How to Overlay Two PDFs for Comparison

Overlaying two PDFs shows you exactly where they differ — every added line, removed annotation, or changed dimension appears in a colour-coded overlay. DrawDiff does this in seconds, entirely in your browser.

Quick Steps

Go to DrawDiff

Open drawdiff.com/app in Chrome, Firefox, Edge, or Safari. No installation or account required.

Load both PDFs

Drag the first PDF onto the Original PDF zone and the second onto the Revised PDF zone. Both zones accept drag-and-drop or click-to-browse. Multi-page PDFs are supported.

Click ▶ Run Compare

The comparison button brightens when both PDFs are loaded. Click it to start. DrawDiff renders both PDFs at high resolution and computes the pixel-level difference.

Read the overlay

Green — original only

Red — revised only

Cyan — unchanged

Zoom with + / − or Alt+scroll. Pan with right-click drag.

Export the overlay

Use the Export section in the sidebar to download JPGs or a combined PDF. The overlay is baked into the image at full render resolution.

What DrawDiff Can Overlay

DrawDiff works with any two PDFs. The comparison is pixel-based — it renders each PDF as a high-resolution image and compares the images pixel by pixel. This means it works regardless of how the PDF was created.

Engineering drawings

CAD exports, plotted drawings, technical specifications — all compared at pixel level

Architectural plans

Floor plans, sections, elevations, site plans across drawing revisions

Contracts and documents

Any PDF containing text — combined with the word-level text diff tool

Scanned drawings

Scanned PDFs compare as images — no text extraction required

When the PDFs Don't Align

If the content in one PDF is shifted, rotated, or at a different scale compared to the other, the direct overlay will highlight the entire shifted area — not just what genuinely changed. Use the Alignment Tool to fix this:

Click ⊕ Alignment Tool after running the initial compare
Draw a crop box over the same view in each pane
Click a reference point (a recognisable feature) on each drawing
Click ▶ Run Aligned Compare

DrawDiff aligns the views on those reference points and reruns the pixel comparison. If the views are at different scales, DrawDiff detects the scale annotation and compensates automatically.

Tip: For very fine detail — small text, hairline rules — increase the Render Scale setting to 6× or 8× before running. This catches changes that would be missed at the default 4× (288 DPI).

How It's Different from Printing and Overlaying

Traditional light-table overlay requires printing both drawings at identical scale, positioning them perfectly by eye, and holding them up to the light. It's time-consuming, requires a plotter, and misses subtle changes in complex areas.

DrawDiff does the same thing digitally in seconds — at higher accuracy, with colour coding to distinguish additions from deletions, and with automated difference detection that numbers every changed region.

Overlay your PDFs now — free, no upload, no account.

▶ Start Overlaying PDFs