ALLYX ONE
100% browser-based · zero uploads

PDF Compare

Upload two PDFs and instantly see what changed — line by line or pixel by pixel. Both files stay on your device the whole time.

PDF A (Original)

Drop PDF here, or browse

PDF up to 100 MB

PDF B (Modified)

Drop PDF here, or browse

PDF up to 100 MB

How It Works

1

Upload both PDFs

Drop one PDF into each panel. Both files are loaded into browser memory via the FileReader API — nothing leaves your device.

2

Choose a mode

Text Diff runs an LCS algorithm on extracted text — fast and precise for document edits. Visual Diff renders pages via PDF.js and compares pixels — catches layout and image changes.

3

Compare in your browser

PDF.js processes both documents page by page using Canvas and WebAssembly. A progress bar tracks each page. All computation stays on your device.

4

Review and export

Navigate page by page. Copy the text diff to your clipboard or download the visual diff image. No server request is made at any point.

Why users love this tool

  • Your files never leave your device — both PDFs are compared locally using PDF.js, never on our servers
  • Two comparison modes — Text Diff for content changes, Visual Diff for layout and formatting changes
  • No account needed — open the page and start comparing straight away

How to Compare Two PDF Files

  1. Drop PDF A (the original) into the left panel and PDF B (the modified version) into the right panel.
  2. Select Text Diff or Visual Diff using the toggle.
  3. Click Compare PDFs — comparison runs page by page in your browser.
  4. Use the page navigator to step through each page. In text mode, copy the diff to clipboard; in visual mode, download the difference image.

Text Diff vs Visual Diff — When to Use Each

Text Diff — best for

  • ✓ Comparing document versions (contracts, reports, proposals)
  • ✓ Spotting wording changes, additions, and deletions
  • ✓ Fast comparison of large multi-page documents
  • ✓ Native (non-scanned) PDFs with selectable text

Visual Diff — best for

  • ✓ Catching layout, formatting, or font changes
  • ✓ Comparing scanned documents or image-heavy PDFs
  • ✓ Spotting moved or resized images, logo changes
  • ✓ Design proofing — compare a draft PDF against a final

Features

  • Your files stay private: Both PDFs are processed entirely in your browser using PDF.js — nothing is ever uploaded or stored on our servers.
  • Free — always: No accounts, no subscriptions, no hidden fees.
  • LCS text diff: The Longest Common Subsequence algorithm finds the minimal set of additions and deletions, matching the approach used by professional diff tools.
  • Inline character highlighting: For paired changed lines, individual characters are highlighted so you can see exactly what changed within a line.
  • Pixel-level visual diff: Pages are rendered at 1.5× scale for clarity. Changed pixels are highlighted in red; unchanged areas are shown desaturated.
  • Multi-page support: All pages are compared with a convenient page-by-page navigator.
  • Up to 100 MB per file: Handles most standard PDF documents.

Frequently Asked Questions

Are my PDF files uploaded to your servers?

No. Your files never leave your device. Both PDFs are loaded into browser memory via the FileReader API and processed entirely by PDF.js running in your browser tab — nothing is sent to our servers or stored anywhere.

What is the difference between Text Diff and Visual Diff?

Text Diff extracts the text content from both PDFs and runs a line-by-line LCS comparison, highlighting added lines in green and removed lines in red. Visual Diff renders each page to a canvas and compares pixels, producing a heatmap of changed areas. Use Text Diff for document edits, Visual Diff for layout and formatting changes.

Does this work with scanned PDFs?

For text diff, scanned PDFs have no embedded text, so the diff will show no differences even if pages look different. Use Visual Diff mode to compare scanned documents pixel-by-pixel.

What is the maximum file size?

Up to 100 MB per PDF. Very large documents with many pages may take longer depending on your device's speed. Visual Diff on high-resolution pages is more CPU-intensive than Text Diff.

Which browsers are supported?

This tool works in all modern browsers — Chrome, Firefox, Edge, and Safari. No plugins or extra software needed.

Can I compare password-protected PDFs?

Password-protected PDFs cannot be compared directly. The tool will show an error if it encounters an encrypted file. You can use the PDF Unlock tool on this site to remove the password first, then compare the unlocked files.

Need to merge your PDFs after reviewing? Try the PDF Merger. Or extract specific pages with the PDF Extract Pages tool.