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CAD / Engineering Drawings
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SumatraPDF can thicken hairline strokes and darken mid-gray lines in CAD and engineering-drawing PDFs, so the drawing stays readable when you zoom out, closer to Acrobat.
Available in version 3.7 or later. PDF only.
Set EngineeringDrawingEnhance = auto to enhance only PDFs that look like CAD drawings. At a glance:
  • Zoom-aware minimum stroke width: hairlines get thicker when zoomed out; no effect when zoomed in.
  • Darker grays: typical CAD-export grays are darkened.
  • Global setting: EngineeringDrawingEnhance = off / auto / on.
  • Per-file toggle: Toggle Engineering Drawing Enhancement in Command Palette, current PDF only.
Text geometry is not changed. Off by default. The Explorer preview pane and the Windows Search filter never apply it.

Enhance drawings in every PDF #

Set EngineeringDrawingEnhance in advanced settings:
EngineeringDrawingEnhance = off
Value Effect
off Never enhance (default)
auto Enhance only when the PDF looks like a CAD drawing
on Enhance every PDF
Change it in any of these ways:
  • Advanced Settings dialog: Settings → Advanced Settings…, filter EngineeringDrawingEnhance, pick off / auto / on, Save. Or Ctrl + K, Advanced Settings... in Command Palette.
  • Command Palette settings: Ctrl + K, type =EngineeringDrawingEnhance, Enter, pick a value. Takes effect immediately, no restart.
  • Settings file: Settings → Open Advanced Settings File…, edit EngineeringDrawingEnhance = …, save.
The setting is global: it applies to every PDF. If a file is already open, reopen it (or use the per-file toggle) so rendering picks up the new value.

Toggle enhancement for one file #

Toggle Engineering Drawing Enhancement (CmdToggleEngineeringDrawingEnhance) flips enhancement for the current PDF only.
The first use flips away from whatever is in effect (so with the default off, it turns enhancement on). After that it alternates on/off for that document.
This override is session-only. It is not saved in FileStates. Closing the file (or the tab) restores the global setting. It does not change EngineeringDrawingEnhance for other files.

Tips #

  • Use auto to leave ordinary PDFs untouched.
  • Use the per-file toggle to compare a drawing with and without enhancement.
  • Use =EngineeringDrawingEnhance in the Command Palette to switch values without a restart.
  • Bind a shortcut to CmdToggleEngineeringDrawingEnhance if you toggle often.

How auto detects drawings #

auto looks for a PDF/E marker, a CAD authoring tool in the document metadata, or content heuristics on the first pages (hairline vectors, raster screenshots of drawings, WPS-style hairline exports).

Global vs per file #

Global EngineeringDrawingEnhance Per-file toggle
Scope All PDFs Current document
Saved Yes, in settings No
Wins Used unless the file has a toggle Beats the global setting for that file

See also #

SumatraPDF documentation
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CAD / Engineering Drawings
md