Compare PDF Files Online — Side-by-Side Diff Viewer Free

Need to find out what changed between two versions of a PDF? Our free PDF comparison tool renders both documents side by side, runs a pixel-level diff analysis across every page, and highlights additions in green and removals in red — giving you an instant visual map of every change. Navigate between pages, jump directly to changed pages, and choose between three view modes: side-by-side, diff-only, or overlay blend.

Everything runs entirely in your browser using PDF.js. Neither document is ever uploaded to any server — making this tool safe for confidential contracts, legal drafts, financial reports, and any sensitive files you need to compare privately.

How to Compare Two PDFs — Step by Step

1. Upload the Original PDF — Drag and drop or click the Original drop zone to upload the first (older) version of your document.
2. Upload the Modified PDF — Upload the second (newer) version to the Modified drop zone.
3. Adjust sensitivity and opacity — Set the diff sensitivity to control how subtle a change needs to be before it is highlighted. Adjust overlay opacity to control how strongly the highlights appear over the page content.
4. Choose a view mode — Select Side by Side (original + diff + modified), Diff Only (just the highlighted differences), or Overlay Blend (both documents semi-transparently merged).
5. Click "Compare PDFs" — The tool scans all pages, builds a change summary, and opens the interactive viewer.
6. Navigate the results — Use Prev / Next to move between pages, or click Next Change to jump directly to the next page with detected differences.

Three View Modes

◫ Side by Side

Displays three panels: the original PDF page on the left, a diff overlay in the center with red/green highlights showing what changed, and the modified PDF page on the right. This is the most informative view — you can simultaneously see where the change is (center), what it was (left), and what it became (right).

🔴 Diff Only

Shows only the diff overlay — a single page with red highlights where content was removed or changed, and green highlights where content was added or changed. Useful for quickly scanning what changed without the distraction of the full page content.

⧉ Overlay Blend

Blends both pages semi-transparently at 50% each. Where the two documents are identical, the content appears clear. Where they differ, you see a visual "ghost" — both versions overlapping. This view is particularly useful for catching layout shifts, text reflows, and image repositioning.

Pixel-Level Diff Engine

How differences are detected

Each page from both PDFs is rendered to a canvas at 1.2× scale. The engine then compares every pixel: for each pixel where the colour difference between the two versions exceeds the sensitivity threshold, it is flagged as changed. Pixels where the original was darker (content removed) are highlighted in red; pixels where the modified version is darker (content added) are highlighted in green.

Diff Score

Each page receives a diff score — the percentage of pixels that changed. The summary bar shows which pages have a score above the sensitivity threshold. The current page's score is shown in the legend bar during navigation.

Sensitivity Control

The sensitivity slider (1–100) sets the minimum per-pixel colour difference required to flag a change. At low sensitivity (1–15), even slight rendering differences or anti-aliasing variations are highlighted. At high sensitivity (50–100), only major content changes are flagged. The default value of 30 works well for most document comparison tasks.

Key Features

• Pixel-level visual diff: Every changed pixel is detected and colour-coded — red for removed content, green for added content.
• Three view modes: Side by Side, Diff Only, and Overlay Blend for different analysis needs.
• Change summary: Total changed pages displayed at the top of the viewer after comparison.
• Jump to next change: One-click navigation to the next page with detected differences — skip unchanged pages instantly.
• Per-page diff score: Percentage of changed pixels shown for each page during navigation.
• Adjustable sensitivity: Fine-tune the threshold to catch subtle or only major changes.
• Adjustable overlay opacity: Control how strongly diff highlights appear over the page content.
• Handles different page counts: If the two PDFs have different numbers of pages, extra pages are compared against a blank white page.
• 100% browser-based: PDF.js renders both documents locally — no data leaves your device.
• No watermarks: This is a viewer tool — no PDF is exported or modified.

Common Use Cases

⚖️ Legal Document Review

Compare two versions of a contract, NDA, or legal brief to find exactly which clauses were added, removed, or modified between drafts — without reading both documents in full.

📋 Policy and Compliance Documents

Regulatory documents, terms of service, and compliance policies are frequently updated. Use the diff viewer to identify exactly what changed between the version you approved and the new version you received.

🎨 Design and Layout Proofing

Compare a design proof with a revised version to catch layout shifts, image repositioning, colour changes, or typography adjustments — particularly useful when the differences are subtle.

📊 Report and Financial Document Comparison

Compare this quarter's financial report with last quarter's to visually spot which figures, tables, or sections changed — faster than manual line-by-line reading.

📚 Academic and Research Document Versioning

Compare submitted drafts with revised versions of research papers, theses, or academic reports to verify that all requested changes were applied correctly.

Frequently Asked Questions

Conclusion

Comparing two versions of a PDF should be visual, instant, and private. With pixel-level diff analysis, three view modes, a change summary, per-page diff scores, jump-to-change navigation, adjustable sensitivity, and zero server uploads, this tool covers every common PDF comparison scenario directly in the browser.

No account. No watermarks. No cost. Scroll up, upload both versions, and see exactly what changed in moments.