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📎 What Is a PDF Underlay?
A PDF underlay is a PDF file attached to your AutoCAD drawing as a reference image, similar to an xref — but for PDF files instead of DWG files. You attach it with the PDFATTACH command, and it appears in your drawing at a position and scale you define. You can trace over it, use it for reference, snap to its geometry, and control its visibility per layer or viewport.
PDF underlays are the modern replacement for the old workflow of printing a PDF, scanning it, and importing the scan as a raster image. They retain vector quality (sharp at any zoom level, when the source PDF is vector), support object snapping, and let you see individual PDF layers.
Introduced in AutoCAD 2010. PDF underlays, PDF snapping, and PDF layer control have been available since AutoCAD 2010. PDFIMPORT (converting PDF vectors to DWG geometry) was added in AutoCAD 2017. All features discussed here are available in AutoCAD 2024.
Common use cases
Client-supplied site plans
- Client sends PDF of existing survey or site plan
- Attach as underlay, trace major features
- Snap to corners and boundaries
- Scale to match real-world coordinates
Architectural base plans
- Architect provides PDF floor plan
- Underlay in structural or services drawing
- Trace walls, columns, grid lines
- Use PDFIMPORT to extract vectors directly
Utility / civil reference
- Attach as-built PDF of underground services
- Overlay proposed design on existing PDF
- Snap to utility centrelines
- Fade underlay for visual clarity
Shop drawings & coordination
- Attach subcontractor shop drawing PDF
- Check clearances against your design
- Turn PDF layers on/off to isolate elements
- Clip to show only relevant area
➕ Attaching a PDF Underlay (PDFATTACH)
Method 1: Command line (fastest)
Command: PDFATTACH
; File selection dialog opens — browse to your PDF
; Select file → click Open
; "Attach PDF Underlay" dialog appears
Method 2: Ribbon
Insert tab
→
Reference panel
→
Attach
The Attach dialog opens. Change the file type filter to "PDF Files (*.pdf)" and browse to your file.
Method 3: Drag and drop
Drag a PDF file from Windows Explorer directly onto the AutoCAD drawing area. AutoCAD will prompt you to choose between attaching as an underlay or importing as geometry (PDFIMPORT). Choose Attach as Underlay.
The Attach PDF Underlay dialog
Attach PDF Underlay
✕
Name:
Site_Plan_Issue_D.pdf ▼ Browse...
Select page to attach:
Page 1 — Site Plan
Page 2 — Notes
Page 3 — Details
Path type:
Relative path ▼
Specify on-screen: Insertion point
Specify on-screen: Scale
Specify on-screen: Rotation
Insertion point:
X:
Y:
Z:
Scale:
(adjust after insertion)
Key settings in the dialog
| Setting | Recommended | Notes |
|---|---|---|
| Select page | Choose the correct page | Multi-page PDFs let you attach one page at a time. Attach the same PDF multiple times to get different pages. |
| Path type | Relative path | Relative path: PDF stays linked as long as both files are in the same relative folder structure. Full path: breaks if the folder moves. No path: AutoCAD searches configured paths. |
| Insertion point | Tick "Specify on-screen" | Lets you click where to place the PDF origin point. Easier than typing coordinates unless you know the exact coordinate to hit. |
| Scale | Leave at 1 initially | Set scale to 1 first, then use the Properties panel or ALIGN command to match the PDF to known geometry. See the Scaling section below. |
| Rotation | 0 unless PDF is rotated | You can adjust rotation later by selecting the underlay and editing in Properties. |
Tip: Insert the PDF at 0,0 with scale 1 first, then scale and position it using known reference points. It's much easier to align after insertion than to guess the correct insertion coordinate.
🎨 Display Controls
Once a PDF is attached, select it and use the PDF Underlay contextual ribbon tab that appears, or right-click → PDF Underlay Properties, to control how it looks.
Fade, Contrast, and Monochrome
| Control | Range | Purpose | Recommended |
|---|---|---|---|
| Fade | 0–80 | Dims the underlay toward the background colour. Higher = lighter/more faded. | 40–60 for background reference; 0 for active tracing |
| Contrast | 0–100 | Adjusts tonal range. Higher = sharper black/white distinction. | 75 for most PDFs; lower for greyscale PDFs with fine tones |
| Monochrome | On/Off | Forces the PDF to display in black and white regardless of its original colours. | On — reduces visual clutter when your drawing uses colour to distinguish layers |
Visual guide to Fade values:
Setting display via Properties panel
; Select the PDF underlay → open Properties
Command: PROPERTIES ; or Ctrl+1
;
; Under "Underlay Adjust" section:
; Fade → enter 0–80
; Contrast → enter 0–100
; Monochrome → Yes / No
; Show underlay → Yes / No (quick on/off)
Show/Hide the boundary frame
The PDF underlay has a visible rectangular border (the frame) that shows its boundary. Control it with PDFFRAME:
Command: PDFFRAME
; 0 = Frame invisible and does not plot
; 1 = Frame visible and plots (default)
; 2 = Frame visible on screen but does NOT plot
Enter new value for PDFFRAME <1>: 2
; Value 2 is recommended — you can select the underlay on screen
; but the frame won't appear on your printed output
🎯 Snapping to PDF Geometry
One of the most powerful features of PDF underlays is the ability to snap (object snap) to geometry embedded in the PDF — endpoints, midpoints, intersections, and more — provided the PDF was created from vector CAD data, not scanned from paper.
Vector vs raster PDF: PDF files fall into two types. A vector PDF (exported directly from CAD, Revit, or Illustrator) contains precise geometry that AutoCAD can snap to. A raster PDF (scanned paper drawing, or a photo embedded in a PDF) contains pixels only — AutoCAD cannot snap to it. If snapping doesn't work, the PDF is raster.
Enable PDF snapping
Command: PDFOSNAPTYPE
; 0 = No snapping to PDF geometry
; 1 = Snap to PDF geometry (endpoints, midpoints, intersections, etc.)
Enter new value for PDFOSNAPTYPE <1>: 1
Also confirm that OSNAP is active (F3 toggles it) and that the relevant snap modes are enabled. Standard snap modes work on PDF geometry:
Snap modes that work on vector PDFs
- ENDpoint — line/arc endpoints
- MIDpoint — midpoint of lines
- INTersection — crossing lines
- PERpendicular — perpendicular to a PDF line
- NEArest — nearest point on a PDF entity
- CENter — centre of PDF circles/arcs
Snap modes that do NOT work on PDFs
- INSert (block insertion points — not applicable)
- NODe (point objects — not in PDFs)
- QUAdrant (quarter points of circles)
- TANgent (tangent points)
- Any snap mode on raster PDFs
Performance tip: PDF snap can slow down drawing on large or complex PDFs. If you notice lag while drawing, set
PDFOSNAPTYPE to 0 temporarily while not actively snapping to the underlay.Testing if your PDF is vector
Zoom in very close to a line in the PDF underlay. If the line remains crisp and sharp at extreme zoom, it's vector. If it becomes pixelated or blurry, it's raster and snapping will not work. You can also try hovering over a line — if you see snap markers appear, it's vector.
✂️ Clipping PDF Underlays (PDFCLIP)
If you only need part of a PDF — for example, just one quadrant of a large site plan — you can clip it to show only the area you want. The rest of the PDF is hidden but not deleted; you can adjust or remove the clip at any time.
Command: PDFCLIP
Select PDF underlay to clip: ; click the underlay
Enter clipping option [ON/OFF/Delete/New boundary] <New boundary>: N
Specify clipping boundary:
[Select polyline/Polygonal/Rectangular/Invert clip] <Rectangular>: R
Specify first corner: ; click first corner
Specify opposite corner: ; click opposite corner
; The PDF is now clipped to that rectangle
Clipping options explained
| Option | What It Does |
|---|---|
Rectangular | Simple four-corner clip — fastest and most common |
Polygonal | Click multiple points to define an irregular clip boundary — useful for site boundaries, road alignments, or irregular shapes |
Select polyline | Use an existing closed polyline in your drawing as the clip boundary — very powerful for complex shapes |
Invert clip | Shows everything OUTSIDE the clip boundary and hides the interior — useful for masking a central area |
ON / OFF | Temporarily shows or hides the clipping boundary without deleting it |
Delete | Removes the clip boundary and restores the full PDF display |
Workflow tip: Draw a closed polyline over the area of the PDF you want to keep. Then run PDFCLIP → Select polyline → click your polyline. This gives you precise control over the clip boundary and lets you adjust it by editing the polyline grips.
Grips clipping: You can also select the underlay and use its grip handles to drag the clip boundary edges — available after a rectangular clip has been applied. This is the quickest way to make small adjustments.
🗂️ PDF Layers
PDFs created with layer information (from AutoCAD, Revit, or other CAD tools) retain those layers. You can turn individual PDF layers on and off inside AutoCAD — without editing the PDF itself.
Accessing PDF layers
1
Select the PDF underlayClick the attached PDF in the drawing.
2
Right-click → PDF Underlay LayersOr use the contextual ribbon tab: PDF Underlay → Layers panel → Edit Layers.
3
Toggle layers on/offThe Underlay Layers dialog shows all layers in the PDF. Click the lightbulb icon to toggle visibility. Click OK.
; Control PDF layer visibility via PDFLAYERS sysvar
Command: PDFLAYERS
; 0 = All layers shown
; 1 = Layers shown according to last saved state in the PDF
; Set to 1 to honour the PDF's own layer visibility settings
Layer limitation: Not all PDFs contain layer data. If a PDF was flattened, printed from a non-CAD application, or created as a raster scan, there will be no layers to toggle. The Underlay Layers dialog will show a single unnamed layer.
📐 Scaling & Positioning a PDF Underlay
Getting the PDF at the right scale and position is the most important step. There are several approaches depending on what information you have.
Method 1: Scale using two known points (ALIGN)
This is the most reliable method when you can identify two points in the PDF that correspond to known coordinates in your drawing.
1
Attach the PDF at scale 1Don't try to scale it during attachment.
2
Type ALIGN → EnterSelect the PDF underlay as the object to align.
3
Click source point 1 on the PDFSnap to a known point — e.g., a boundary corner visible in both the PDF and your drawing.
4
Click destination point 1 in your drawingClick or type the coordinate where that point should land.
5
Click source point 2 and destination point 2A second pair of points lets AutoCAD calculate both scale and rotation simultaneously.
6
When prompted "Scale objects based on alignment points?" → YesAutoCAD scales and rotates the underlay to match both point pairs perfectly.
Method 2: Scale from a known dimension on the PDF
If you know the real-world length of something visible on the PDF (e.g., a road segment, a room dimension, a scale bar):
1
Attach the PDF at scale 1
2
Draw a temporary line between two known points on the PDFUse SNAP to the PDF endpoints.
3
Check the line's length with LIST or DISTCompare to the known real-world length.
4
Calculate scale factor: real length ÷ current lengthExample: real length = 50m, current length = 0.1764m → scale factor = 283.4
5
Select the underlay → SCALE → pick base point → enter scale factorThe underlay resizes to match real-world dimensions.
Method 3: Properties panel
Select the underlay → open Properties (Ctrl+1). Under Geometry you can directly set X scale, Y scale, rotation, and insertion X/Y/Z coordinates — useful for fine-tuning after initial placement.
PDF scale bars are unreliable. Printed PDFs can be slightly distorted from paper stretching, scanning, or printer miscalibration. Always verify your scale using at least two known distances if possible. A scale bar that looks correct might be 1–3% off, which adds up significantly on large site plans.
⬇️ PDFIMPORT — Converting PDF to DWG Geometry
If you need to work with the PDF geometry as actual AutoCAD objects (lines, polylines, arcs, text) rather than just tracing over an underlay, use PDFIMPORT. This converts the vector content of a PDF directly into DWG entities. AutoCAD 2017+
PDFIMPORT only works on vector PDFs. Raster (scanned) PDFs will not yield usable geometry — you'll get a raster image object instead. TrueType text in the PDF may import as text or as polyline outlines depending on how the PDF was created.
Running PDFIMPORT
Command: PDFIMPORT
; Two modes: import from file, or import from an already-attached underlay
; Option A: Import from an attached underlay Select PDF underlay to import or [File]: ; click the underlay in the drawing
; Option B: Import directly from a PDF file (no underlay needed) Select PDF underlay to import or [File]: F ; File dialog opens — browse to the PDF
; Option A: Import from an attached underlay Select PDF underlay to import or [File]: ; click the underlay in the drawing
; Option B: Import directly from a PDF file (no underlay needed) Select PDF underlay to import or [File]: F ; File dialog opens — browse to the PDF
PDFIMPORT dialog settings
| Option | Recommended | Notes |
|---|---|---|
| PDF Page | Select the correct page | Multi-page PDFs let you choose which page to import. |
| Import as | AutoCAD objects | "AutoCAD objects" converts to native lines, arcs, polylines, text. "Static drawing" imports as a block. |
| Location | Specify insertion point on-screen | Or type 0,0 to import at the drawing origin. |
| Layers | Use PDF layers | Preserves the PDF's layer structure in AutoCAD. "All on one layer" is simpler but loses organisation. |
| Import settings | Vector geometry: On; Solid fills: Off (usually); TrueType text: On | Solid fills can import as hatches — usually unwanted unless you need the fills. TrueType text imports as editable AutoCAD text. |
After importing: cleaning up the geometry
PDFIMPORT geometry often needs cleanup — duplicate lines, very short line fragments, disjointed polylines. Useful commands after import:
; Join fragmented lines into polylines
Command: PEDIT ; select multiple → Join
; or use JOIN command:
Command: JOIN
; Remove duplicate/overlapping geometry Command: OVERKILL ; OVERKILL removes duplicate and overlapping lines, arcs, and polylines
; Purge unused layers and blocks from import Command: PURGE
; Remove duplicate/overlapping geometry Command: OVERKILL ; OVERKILL removes duplicate and overlapping lines, arcs, and polylines
; Purge unused layers and blocks from import Command: PURGE
PDFIMPORT vs tracing: For a simple site plan with 20 key elements, tracing over the underlay is faster than cleaning up PDFIMPORT results. For a complex floor plan with hundreds of walls, PDFIMPORT saves hours. Use judgement — the messier and more complex the import, the more time you'll spend on cleanup.
📋 Managing PDF Underlays
PDF underlays are managed through the same External References palette as DWG xrefs, DGN underlays, and image references.
Command: XREF ; or XR
; External References palette opens
; PDF underlays appear with a PDF icon alongside xrefs and images
Actions from the Xref palette
| Action | How | Notes |
|---|---|---|
| Attach new PDF | Click the paperclip/attach icon → PDF Underlay | Same as running PDFATTACH |
| Reload | Right-click underlay → Reload | Re-reads the PDF from disk — use after the PDF file has been updated by the sender |
| Unload | Right-click → Unload | Keeps the reference in the drawing but stops displaying it. Faster than detach if you may need it again. Reduces screen refresh load. |
| Detach | Right-click → Detach | Permanently removes the PDF reference from the drawing. Can't be undone after saving. |
| Change path | Right-click → Change Path | Re-links a missing PDF to its new location. Fix "not found" errors here. |
PDF underlays and eTransmit
When you send a drawing with a PDF underlay to a colleague or client, the PDF file must be included. Use ETRANSMIT to package everything together:
Command: ETRANSMIT
; AutoCAD bundles the DWG + all PDF underlays + xrefs + fonts into a zip
; Recipient can open the DWG and all underlays will resolve automatically
⚖️ PDF Underlay vs Image Reference vs DGN Underlay
📎 PDF Underlay
- Vector-sharp at any zoom (if source is vector)
- Snappable geometry (vector PDFs)
- PDF layer control
- Fade, contrast, monochrome controls
- Multi-page support
- Source file must stay linked
- PDFIMPORT converts to DWG objects
🖼 Raster Image (IMAGEATTACH)
- Pixelates when zoomed in
- No geometry snapping
- Works for scanned drawings, photos, site aerials
- Brightness/contrast/fade controls
- Can be clipped
- Better for non-CAD imagery
- Formats: JPG, PNG, TIFF, BMP, etc.
📐 DGN Underlay (DGNATTACH)
- MicroStation native format
- Vector-sharp, snappable
- Level (layer) control
- Best when client/authority uses MicroStation
- Common in civil, survey, transport
- See DGN Reference page for full detail
Rule of thumb: Use a PDF underlay when the source is a vector PDF from another CAD tool. Use an image reference for scanned drawings, aerial photos, or non-CAD graphics. Use a DGN underlay when working with MicroStation files from surveyors, councils, or infrastructure authorities.
⌨️ System Variables
PDFFRAME
0 = Invisible | 1 = Visible + plots | 2 = Visible, no plot
Controls the display of the PDF boundary rectangle. Value 2 is recommended — visible for selection on screen, hidden on printed output.
PDFOSNAPTYPE
0 = No snap | 1 = Snap to geometry
Enables or disables object snapping to PDF underlay geometry. Set to 0 temporarily if PDF snapping causes slowdowns on large files.
PDFLAYERS
0 = All layers shown | 1 = Per PDF saved state
Controls how PDF layer visibility is initialised when the underlay is attached. Value 1 honours the PDF's own saved layer visibility.
PDFSHX
0 = Off | 1 = On
When On, SHX (AutoCAD custom font) text from a PDF export is stored in a data file that PDFIMPORT can read back as text. Leave Off unless specifically working with SHX text recovery.
UOSNAP
0 = Off | 1 = On
Master toggle for object snapping to all underlay types (PDF, DGN, DWF). If PDF snapping isn't working, check this is set to 1.
IMAGEFRAME
0 / 1 / 2
Controls frame display for raster image references (not PDF underlays). Same values as PDFFRAME. Separate from PDFFRAME — both can be set independently.
🔧 Troubleshooting
| Problem | Likely Cause | Fix |
|---|---|---|
| PDF underlay shows as missing / not found | The PDF file has been moved, renamed, or is on a network drive that's not mapped | Open XREF palette → right-click the PDF → Change Path → browse to the new location. Or map the network drive and reload. |
| Cannot snap to PDF geometry | PDF is raster (scanned), PDFOSNAPTYPE is 0, or UOSNAP is 0 | Check: set PDFOSNAPTYPE=1 and UOSNAP=1. If still no snap, the PDF is raster and snapping is not possible — you must trace manually or use PDFIMPORT (which will import as an image). |
| PDF appears blurry or pixelated | PDF is raster, or display resolution is limited | Vector PDFs are always sharp. Raster PDFs are limited by the DPI of the scan. Check the PDF source — if scanned, get a vector export from the originator. |
| PDF is attached but invisible in the drawing | The underlay layer is frozen or off, or the underlay is on a layer not visible in the current viewport | Open Layer Manager and check if the PDF's layer is on and thawed. Also check PDFFRAME — if set to 0, no frame is shown, making it hard to locate. Set PDFFRAME=1 temporarily. |
| PDF frame appears in the plot/PDF output | PDFFRAME is set to 1 (visible and plots) | Set PDFFRAME=2 — this keeps the frame visible on screen for selection but excludes it from print output. |
| PDF underlay is at wrong scale | Inserted with incorrect scale, or PDF was created at a different unit | Use ALIGN with two known reference points to rescale. Or check the PDF's unit system — a PDF drawn in feet will be 304.8× too large in a metric drawing. |
| PDFIMPORT produces hundreds of tiny line segments | PDF was created with complex splines or geometry broken into many small vectors | Run OVERKILL to remove duplicates, then PEDIT → Join to reassemble fragmented lines. Consider whether manual tracing would be more efficient. |
| Text from PDFIMPORT is unreadable or imports as outlines | PDF uses SHX fonts or fonts were embedded as outlines (flattened) | SHX text appears as polyline outlines. This is a PDF limitation — AutoCAD cannot recover the original text. Retype manually or try PDFSHX=1 if the PDF was exported from AutoCAD with SHX data. |
| Performance slow with PDF attached | Large or complex PDF, or PDF snap enabled on a complex underlay | Set PDFOSNAPTYPE=0 when not actively snapping. Unload the underlay (XREF palette → Unload) when not needed. Clip the PDF to only the area you need. |
| Colleague can't see the PDF underlay | PDF file not sent with the drawing | Use ETRANSMIT to package the DWG and all references together. Never send just the DWG file when it has underlays. |
✅ Best Practices
1. Use relative paths. When attaching PDFs, always choose "Relative path" in the attachment dialog. This keeps the PDF-to-DWG link intact as long as both files sit in the same relative folder structure — even when the whole project folder moves to a different drive or computer.
2. Set PDFFRAME to 2. Set this once in your drawing template. The underlay boundary is visible on screen (so you can select and manipulate it) but absent from all printed and PDF output.
3. Fade to 50, Monochrome On. This combination keeps the underlay visible without competing with your active drawing linework. Adjust fade up or down per project — finer reference PDFs can go lower.
4. Attach on a dedicated layer. Put all PDF underlays on a layer like
PDF-REF or UNDERLAY-PDF. This makes it trivial to freeze or lock all underlays at once, or to exclude them from a selection set. Colour the layer grey so it's obviously reference-only.5. Get a vector PDF from the source. If a client or consultant sends you a scanned PDF, ask them for the original CAD export. A vector PDF is dramatically more useful — you can snap to it, import it, and control its layers.
6. Clip aggressively. A 20-page A1 site plan PDF attached at full size is slow to display and hard to work with. Clip it to just the area you're working in. Your drawings will be faster and cleaner.
7. Use PDFIMPORT for redraw work, underlay for reference. If you're redesigning an area and need to reproduce existing features as DWG objects, PDFIMPORT + OVERKILL is worth the cleanup effort. If you just need visual reference while drawing something new, an underlay is faster and tidier.
8. Always ETRANSMIT before sending. A drawing that references a PDF on your Q: drive will show broken references on the recipient's computer. ETRANSMIT packages everything — DWG, PDFs, xrefs, fonts — into a zip the recipient can open and have everything resolve correctly.