Guide

AutoCAD PDF Underlay : A PDF file inserted into a drawing as a reference

If you need a complete, beginner-friendly guide to using PDF underlays in AutoCAD, this article explains what a PDF underlay is, why and when to use it, step‑by‑step instructions to attach, edit, clip and convert PDFs, alternative methods, common errors and fixes, plus practical tips to improve productivity.

What is a PDF underlay?

A PDF underlay in AutoCAD is a PDF file inserted into a drawing as a reference. Unlike imported geometry, a PDF underlay remains a non‑editable reference (a raster/vector snapshot) that you can use to trace, measure, align, or compare with your CAD geometry. AutoCAD treats attached PDFs similarly to external references (XREFs): you can move, scale, clip, manage visibility and update or detach them.

Why use a PDF underlay?

  • To trace existing drawings when only a PDF is available.
  • To verify dimensions or compare revisions without converting the entire PDF.
  • To use as a visual reference for site plans, scanned sheets, vendor drawings, or architectural layouts.
  • To preserve original content while working on a clean CAD model (underlay remains unchanged).
  • To import geometry selectively when you need editable entities (via conversion).

Key differences: PDF underlay vs PDF import

  • PDF Underlay (Attach / PDFATTACH): Adds the PDF as a reference. Best for tracing and visual comparison. Non‑editable as native CAD objects. Lower risk of creating messy geometry.
  • PDF Import (PDFIMPORT): Converts vector PDF content into editable lines, polylines, text and hatches. Use when you need to edit geometry, but expect conversion limitations (font substitution, grouped polylines).
  • Choose underlay to work faster and keep file size small; choose import when you need native AutoCAD entities.

How to attach a PDF underlay (step-by-step)

  1. Open your drawing in AutoCAD.
  2. On the ribbon, go to the Insert tab → Attach (or type PDFATTACH on the command line).
  3. In the file dialog, select the PDF file and click Open.
  4. In the PDF Underlay dialog:
    • Choose the page to attach (for multi‑page PDFs).
    • Set Insertion Point, Scale, and Rotation (or check “Specify On‑Screen” to place interactively).
    • Click OK.
  5. Click in the drawing to place the underlay (if you chose on‑screen placement). Use command line prompts or the dialog to confirm scale and rotation.

Tips:

  • If the PDF appears too large/small, detach and reattach with a different scale or use the Properties palette to adjust the Scale X/Y.
  • For precision, use a known distance on the PDF and scale it by measuring and applying a scale factor.

How to clip and control the visible area

  • Use the PDFCLIP command (type PDFCLIP) to create a rectangular or polygonal clipping boundary that hides parts of the underlay.
  • Select the underlay, then define the clip boundary by drawing a rectangle or polygon. Everything outside the clip is hidden.
  • To remove clipping, select the underlay and choose Remove Clipping from the right‑click menu or use PDFCLIP and choose the remove option.
  • You can also move or edit the clip boundary grips after creation.

How to change appearance and visibility

  • Select the PDF underlay and open the Properties palette:
    • Fade: Increase to make the underlay lighter for tracing.
    • Contrast and Monochrome: Adjust for better readability.
    • Layer: Place the underlay on a dedicated locked layer so it cannot be accidentally edited or moved.
    • Plot/No Plot: Control whether the underlay prints (set layer plotting off if you don’t want it printed).
  • Use the External References (XREF) palette to reload, detach, bind, or open the source PDF.
  • The Underlay frame can be toggled (type UNDERLAYFRAME) to show/hide the blue frame around the underlay.

Converting a PDF underlay into editable CAD entities

When you need editable lines, text or hatches:

  1. Use the PDFIMPORT command (type PDFIMPORT).
  2. Select the PDF file and the target page.
  3. In the import options, choose whether to:
    • Import vector geometry (lines, polylines),
    • Import text as text (if font info allows),
    • Import fills and rasters.
  4. Review the imported layers and clean up unwanted entities (erase duplicates, join polylines, correct text styles).

Notes:

  • Imported data may create many small segments; use PEDIT / JOIN and cleanup routines.
  • Some text may come in as geometry rather than text — manual re‑typing might be needed.

Common workflows and examples

  • Tracing an architectural floor plan: Attach PDF → set fade 70% → lock underlay layer → trace walls with polylines → use OSNAP on drawn geometry for accuracy.
  • Verifying site boundary: Attach georeferenced PDF (survey) → scale to known distance → create new boundaries in CAD → compare with new survey.
  • Converting vendor drawing: Use PDFIMPORT → separate by layer → correct scale and units → clean up and standardize lineweights.

Alternative methods

  • Convert PDF to DWG with a third‑party tool (e.g., specialized converters) and then attach or import the resulting DWG.
  • Rasterize the PDF (export to TIFF/JPEG) and use IMAGEATTACH if the PDF contains only raster content. Note: raster images may lose vector clarity.
  • Use Adobe Acrobat or online tools to extract specific pages before attaching to keep file size small.

Common errors and fixes

  • PDF appears extremely large or tiny:
    • Cause: Units mismatch or wrong scale at attach.
    • Fix: Reattach with correct scale, or use Properties to adjust Scale X/Y. Measure a known dimension in the PDF and apply a scale factor.
  • PDF underlay is not selectable:
    • Cause: Underlay may be on a locked or frozen layer, or selection filters block it.
    • Fix: Unlock/unfreeze the layer; ensure selection cycling is enabled, or use the External References palette to manage.
  • PDF text looks wrong or missing after import:
    • Cause: Fonts not embedded or incompatible (SHX vs TrueType).
    • Fix: Import as geometry or retype text after import; use original files if available.
  • Clipped area still shows content:
    • Cause: Clip boundary not applied correctly or underlay has multiple objects/pages.
    • Fix: Reapply PDFCLIP or reattach the correct page. Verify that the clip is active.
  • Underlay not plotting or printing:
    • Cause: Underlay layer set to No Plot, or PDF underlays sometimes default to non‑plot.
    • Fix: Turn layer plotting on or export the desired content to CAD entities and plot those layers.

Practical tips and productivity tricks

  • Always place underlays on a separate, named layer (e.g., PDF_UNDERLAY) and lock it after positioning.
  • Use Fade to 70–90% for easier tracing; reduce eye strain and distinguish new CAD lines.
  • If working from multi‑page PDFs, extract only the needed pages to reduce file size and speed performance.
  • For precise scaling, identify two known points on the PDF and use ALIGN or the scale feature to set exact scale.
  • Use External References palette to manage attached PDFs, reload updated PDFs, or detach obsolete files.
  • When converting, plan for cleanup time: imported geometry often needs simplification and layer reorganization.

FAQ

How do I snap (OSNAP) to points on a PDF underlay?

AutoCAD does not always support object snaps directly to PDF underlays as to native geometry. For accurate snapping, either trace the underlay into CAD entities first or use measurement and alignment tools (e.g., ALIGN, SCALE) with known reference points in the PDF.

Can I attach only one page of a multi‑page PDF?

Yes. When attaching a PDF with PDFATTACH, the dialog asks you to choose the page to attach. Alternatively, extract the page as a separate PDF before attaching.

Will a PDF underlay increase my DWG file size?

No — attached PDF underlays reference the external file and do not embed its full content into the DWG. However, performance may degrade if the PDF is very large or placed at very high resolution. Importing converts content into DWG entities and will increase file size.

How can I convert SHX text from a PDF into editable text in AutoCAD?

PDF conversion may not preserve SHX text as editable text. Use PDFIMPORT to try to import text, but expect some text to come in as geometry. The most reliable method is manual re‑creation of text using appropriate fonts.

What is the difference between turning off the underlay frame and clipping?

Turning off the underlay frame hides the visible rectangle border around the PDF; it does not remove any PDF content. Clipping actually limits which part of the underlay is visible by creating a boundary that hides content outside it.

My PDF underlay is blurry when zoomed in — why?

Vector content should remain crisp. Blurriness often comes from raster PDFs or when AutoCAD displays a lower resolution preview to improve performance. Convert to vector where possible or import the PDF if you need crisp, editable lines.

Can I print a PDF underlay with my drawing?

Yes — provided the underlay layer is set to plot, and the underlay itself is not set to non‑plot. Check layer plotting settings and the underlay properties before printing.

Does AutoCAD automatically update an edited source PDF?

If the source PDF file is changed, go to the External References palette and Reload the underlay, or right‑click the underlay and choose Reload. AutoCAD does not always auto‑refresh active drawings to avoid unexpected changes.

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