PDF Diff Viewer
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What is PDF Diff?
PDF Diff compares two PDF files visually by rendering each page onto a canvas and calculating pixel-level differences. The tool highlights changed areas in red, producing a diff image that shows exactly where the two documents diverge. It compares page 1 of each document by default.
The implementation uses pdf.js to render both pages at 2x scale onto canvas elements, then iterates through every pixel to compute the color difference. Pixels that differ by more than a threshold of 30 are marked in red; unchanged pixels are shown at reduced opacity. The result is a single PNG diff image.
How to Use PDF Diff
- Upload the first PDF (original) by dropping it onto the upload zone.
- Provide the second PDF (revised) using the second PDF option.
- Click Compare PDFs. The tool renders both pages and computes the visual diff.
- Download the result as
diff.png— a side-by-side visual comparison with differences highlighted in red.
Why Use PDF Diff?
When reviewing contract revisions, comparing draft versions, or verifying that edits were applied correctly, a visual diff is faster than reading both documents word by word. PDF Diff shows you exactly which regions changed, making it easy to spot unauthorized modifications, layout shifts, or missing content. Unlike text-based diff tools, it catches formatting and visual changes that don't affect the raw text.
Privacy & Security
Your files never leave your browser. Every step — rendering both PDFs, computing pixel differences, and generating the diff image — happens on your own device. Nothing is uploaded to Racira's servers, no copies are stored in the cloud, and no file data is transmitted at any point.
Frequently Asked Questions
Can I compare specific pages?
The current implementation compares page 1 of each document. For multi-page comparison, you can split each PDF into individual pages, then compare specific page pairs using this tool.
What does red mean in the diff?
Red pixels indicate areas where the two documents differ visually. The threshold of 30 (on a 0–255 per-channel scale) catches even subtle changes like font rendering differences or slight layout shifts. Unchanged areas appear as faded versions of the original.