Compare PDF
View two PDF files side by side to easily spot differences.
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This tool shows two PDFs side by side, page by page, and can overlay a red highlight showing where the two pages differ — entirely in your browser, using pdf.js to render each page to a canvas. The comparison itself is purely visual: both pages are rendered as images at the same zoom level, then compared pixel by pixel, and any pixel whose color differs enough between the two is painted translucent red on top of the left (“Original”) page.
Because the check is pixel-based rather than text-based, it has no idea what a word, sentence, or paragraph is — it can't tell you that word X changed to word Y, only that the pixels at that spot don't match. It compares one page at a time at the page number you're currently viewing, and the two pages are aligned to their top-left corner, so it works best when both PDFs share the same page size and layout; a page where text reflowed or shifted down will show a wide band of red even if the actual wording barely changed.
How it works
1. Upload both PDFs
Drop the original into the left panel and the modified version into the right panel (each accepts one .pdf up to 100 MB). Each side loads and renders independently.
2. Step through pages
The toolbar's Page X / N control moves both documents to the same page number together. If one PDF has fewer pages than the other, the shorter side shows “This page does not exist in this document” once you go past its last page.
3. Zoom and sync scroll
Zoom runs from 50% to 300% in 25% steps and applies to both panels. Sync Scroll is on by default, so scrolling either panel moves the other to match — toggle it off with the Sync Scroll button if you want to inspect each independently.
4. Turn on Highlight Differences
This button is disabled until both files are loaded. Once on, a semi-transparent red overlay appears on the left page wherever its rendered pixels differ from the right page's pixels at the same position, recalculated every time you change page or zoom.
When you'd use it
Checking a contract revision before signing
When a counterparty sends back a revised contract, viewing both versions side by side with differences highlighted quickly draws your eye to the clauses that changed, so you're not re-reading the entire document to find what moved.
Verifying a reprinted or re-exported document matches
After re-exporting a PDF from a different tool or system, comparing it against the original page by page confirms nothing shifted or dropped, since a re-export bug often changes spacing or layout that's easy to miss by eye alone.
Spot-checking a form template after an edit
If you or someone else edited a fillable form or letterhead template, comparing the new version against the last known-good one highlights any accidental layout change on a specific page.
Comparing two scans of the same page
For two scans of what should be the same physical page, the pixel overlay highlights whether they're genuinely identical or something — a stamp, a signature, a stray mark — differs between them.
What “different” means here: pixels, not words
The Highlight Differences overlay is computed by rendering both pages to canvases at the current zoom, then comparing each pixel's red, green, blue and alpha values one by one. If the combined difference across those four channels exceeds a set threshold, that pixel is painted red at roughly 59% opacity; otherwise it's left transparent. There's no OCR, no text extraction, and no concept of a sentence or word — a single character changing lights up only the pixels that character occupies, while a font-rendering difference or a scan artifact lights up just as readily as a real content change.
Because comparison is pixel-position based, the two pages need to line up for the highlight to mean what you expect. The tool's own on-screen note puts it plainly: it works best when both pages share the same size and layout, and a shifted, reflowed, or differently sized page can light up large areas even when the underlying text is similar.
One page at a time, not a whole-document scan
The comparison only ever looks at whichever page number is currently displayed — there's no “scan the whole document and list every page with a difference” feature. For a long document, that means paging through it yourself, turning on Highlight Differences (or watching for a page whose content visibly shifted), and checking each page in turn rather than getting a single summary of every changed page.
Frequently asked questions
Does this tool compare text or just how the pages look?
Just how they look. It renders both pages as images and highlights pixels that differ in color — there's no text extraction or word-level comparison, so it can't tell you what word changed, only where the rendered pixels don't match.
Why is a whole area highlighted red even though only one line of text changed?
The two pages are compared pixel-for-pixel aligned to their top-left corner. If text reflowed or a paragraph got longer or shorter, everything below that point shifts position and shows up as a difference, even though the wording itself may be nearly identical.
Does it compare every page at once?
No, it compares one page at a time — whichever page number you're currently viewing in both panels. There's no summary listing which pages differ; you page through and check each one.
What if the two PDFs have different numbers of pages?
You can still page through both; once you go past the last page of the shorter document, that panel shows “This page does not exist in this document” instead of a blank or error.
What's the largest PDF I can compare here?
Each PDF is capped at 100 MB, checked as soon as you select the file, and up to one file per panel.
Are my files uploaded anywhere?
No. Both PDFs are rendered and compared entirely in your browser using pdf.js; neither file is sent to a server.