How to Compare Two Revisions of an Engineering Drawing
When Revision B arrives, you need to know exactly what changed from Revision A. This guide walks through the complete process using DrawDiff — from loading the PDFs to exporting a numbered redline for your records.
Standard Comparison — Step by Step
Open DrawDiff
Go to drawdiff.com/app in any modern browser. No account or installation required.
Load the original drawing
Drag your original (earlier revision) PDF onto the Original PDF drop zone in the left sidebar, or click the zone to browse for the file. The filename and page count will confirm it loaded.
Load the revised drawing
Drag the revised PDF onto the Revised PDF drop zone. The ▶ Run Compare button will brighten once both PDFs are loaded.
Run Compare
Click ▶ Run Compare. DrawDiff renders both PDFs at the configured render scale (default 4× = 288 DPI) and produces a pixel-accurate overlay. A progress bar shows status for multi-page documents.
Read the overlay
The result is a colour-coded overlay showing every pixel-level difference:
Zoom in with + or scroll with Alt+scroll. Pan by right-click dragging. Use the FIT / W / H buttons to reset the view.
Find and number the differences
Click ◎ Find Differences in the toolbar. DrawDiff automatically clusters changed pixels into regions, assigns each a number, and lists them in a right-hand panel. Click any region in the list to jump to it on the drawing.
Use the toggle buttons to show/hide the numbered boxes and adjust the box colour to suit your background. This numbered list can serve as the basis for your revision change log.
Inspect regions in detail
Hold Ctrl and drag over any area to open the Region Inspector — a three-panel popup showing the overlay, original, and revised versions of that exact region at full resolution. Use this to understand exactly what changed in a complex area.
Export
Use the Export section in the sidebar to save JPGs (one per page) or a single PDF of all redlined sheets. Set a filename prefix that matches your drawing number. The ◎ Find Differences export bakes the numbered overlay into the image at full resolution.
When the View Has Shifted Between Revisions
Sometimes a drawing is re-detailed between revisions, moving the position of a view on the sheet, or changing its scale. A direct pixel comparison will show the entire view as "different" even if the content hasn't changed — just its position.
The Alignment Tool solves this. Click ⊕ Alignment Tool (enabled after running a compare), draw a crop box over the same view on each drawing, then click a recognisable reference point on each (a corner, a hole centre, a datum). DrawDiff aligns the views on those points before running the comparison.
If the views are also at different scales, DrawDiff reads the scale annotation from the drawing (SCALE 1:2, SCALE 1:50, etc.) and compensates automatically. You can also enter the scale factor manually.
Tips for Better Results
- Export at consistent quality — if possible, export both PDFs from the same CAD package at the same settings to reduce rendering artefacts
- Adjust the Diff Threshold — if you're seeing too much noise from anti-aliasing or compression, increase the threshold slider slightly
- Use the background setting — for drawings on a white background, the White Background mode reduces background noise in the comparison
- Multi-page packages — compare the complete drawing package, not individual sheets, to compare all sheets in one run
Ready to try it? DrawDiff is free — no account, no upload.
▶ Compare Drawings Now