How to compare two PDF versions side by side in your browser
Drop a Before and an After PDF to see exactly what changed between them — side-by-side pages, a pixel-level overlay, or a word-level text diff — with neither file ever leaving your browser.
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Compare puts two versions of a PDF next to each other and shows what changed — visually, pixel by pixel, or word by word. Both files render entirely in your browser; nothing is uploaded, so it's safe to use on contracts, leases, and other documents you can't share.
Before you start
- Compare is free and doesn't need an account.
- Everything runs locally. Unlike most online comparison tools, AttachKit never uploads either PDF — you can open your browser's DevTools network tab and watch.
- If either PDF is password-protected (it asks for a password to open), remove the password first with Unlock. The error message links there if you hit it.
- The Text diff view needs real, selectable text in the PDF. Scanned pages (photos of paper) have no text layer — for those, use the Overlay diff view, or run the scan through Searchable (OCR) first.
- Compare is a viewer, not an editor: it doesn't produce a marked-up output file. You read the differences on screen.
Steps
- Open Compare.
- Drop the older version into the Before (A) slot and the newer one into After (B) — or click each slot to pick a file. (No two versions to hand? Click Try a sample before/after pair to load a short generated agreement with a couple of edited clauses, built right in your browser.)
- Once both files load, the workspace opens with the two filenames in the header and a page-count summary. Pick a view:
| View | What it shows | Best for |
|---|---|---|
| Side-by-side | Both pages rendered at the same size, next to each other | Eyeballing layout and skimming |
| Overlay diff | Page A with a red wash over every pixel that differs from B, plus a "% of pixels differ" figure | Scans, stamps, signatures, layout shifts — anything visual |
| Text diff | Both pages' text merged into one readable paragraph: added words highlighted, removed words struck through, with +N added / −N removed counts | Contract redlines and wording changes |
- Step through the document with ← Prev and Next →. The page counter (for example "Page 2 / 5") and the active view stay in sync — page 3 of A is always compared against page 3 of B.
- Use Swap to flip which file counts as Before and which as After, or click Replace on a slot in the drop screen to load a different file. Start over clears both.
Notes on how the diffs work
- The overlay comparison is tolerant of tiny rendering differences (anti-aliased text edges), so a re-saved but unchanged page shouldn't light up red.
- The text diff is word-level: it finds the longest common in-order sequence of words between the two pages and marks everything else as added or removed. If nothing changed, it says No text differences on this page.
- If the two PDFs have different page counts, Compare walks the first N pages (the shorter document's count) and notes which file has extra pages past the comparison range — the header says, for example, "A has 4, B has 6 — comparing first 4."
- Both documents live only in your browser's memory. If you try to close or refresh the tab while files are loaded, the browser warns you first — there's nothing saved to recover.
Result
You see exactly what changed between the two versions, page by page: a red wash over visual differences with a percentage, or an inline redline with added/removed word counts. Nothing was uploaded and nothing is stored — when you close the tab, the comparison is gone.
Related
- Compare troubleshooting — password errors, scanned PDFs, everything-marked-red overlays
- Unlock — remove a password so a PDF can be compared
- Searchable (OCR) — add a text layer to a scan so the text diff has something to read
Related
Still stuck? Contact support →