Compare PDF
Compare PDF
View two PDF files side by side to easily spot differences.
- Runs in your browser
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- Free, no daily limits
- 1Add your file
- 2Choose settings
- 3Download
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It is processed inside this browser tab and never uploaded, and it is gone when you close the page.
Good to know
- Highlight Differences compares the two pages pixel by pixel.
- It compares the page you are viewing, one page at a time.
Last updated
Compare PDF provides a side-by-side visual diff tool that renders two PDF documents synchronously in your web browser. Using pdf.js, each document page is drawn onto an HTML5 canvas at matching zoom levels (50% to 300%). When you toggle 'Highlight Differences', the engine performs a pixel-by-pixel mathematical difference across the red, green, blue, and alpha channels, overlaying a translucent red mask over discrepancies on the left ('Original') page.
Because comparison is strictly visual rather than semantic, the tool detects spatial image variations rather than parsed textual sentences. If text reflows or line wraps shift paragraphs downward, the entire affected area reflects a pixel difference. If errors or layout issues are detected during your review, you can edit and correct them using /pdf/edit.
How it works
1. Upload both documents
Select your reference PDF into the left ('Original') viewer and the modified revision into the right ('Modified') viewer. Files up to 100 MB load locally in browser memory.
2. Synchronise navigation and zoom
Use the shared Page X / N control to flip through corresponding pages. Zoom from 50% to 300% in 25% increments; Sync Scroll keeps viewing positions locked.
3. Enable Highlight Differences
Click Highlight Differences to activate the pixel comparison shader. A semi-transparent red overlay highlights all canvas coordinates where pixel values differ.
4. Audit changes and apply corrections
Inspect highlighted clauses, numbers, or stamps. If revisions require adjustments, open the file in Edit PDF to update annotations or layout.
When you'd use it
Verifying supplier contract amendments
Compare vendor counter-proposals against your standard master services agreement to ensure clauses, liability terms, or payment dates have not been modified without consent.
Checking desktop publishing and print exports
When re-exporting marketing brochures or magazines from InDesign, compare new PDF exports against earlier proofs to catch displaced text frames or missing vector logos.
Quality checking scanned annexures against originals
Place original digital PDFs beside scanned paper copies to identify physical stamping, missing initial signatures, or scanner skew.
Auditing official form template updates
Compare newly revised government or institutional forms against previous versions to locate updated statutory wording or modified input boxes.
How pixel diffing operates: Visual pixels versus semantic text
In CompareClient.tsx, the client renders both pages to offscreen canvas contexts at the active magnification level. It extracts the raw RGBA pixel arrays and evaluates the Manhattan color distance for each coordinate pair: Math.abs(r1 - r2) + Math.abs(g1 - g2) + Math.abs(b1 - b2).
If this color difference exceeds the sensitivity threshold, the pixel is painted red at approximately 59% opacity. What this means in practice:
- **What it detects:** Color shifts, replaced logos, altered signatures, changed dates, deleted sentences, inserted lines, and spatial layout movement.
- **What it cannot tell you:** The engine does not perform word-level AST diffing. It cannot tell you that word 'Foo' was replaced with 'Bar' in the exact same spot: it simply flags that the pixels forming the glyphs do not match.
- **Layout reflow impact:** If an added sentence pushes subsequent paragraphs down the page, all lines below the insertion point shift to different coordinates, causing the entire lower page to illuminate red. For optimal results, compare documents that maintain identical grid layouts.
Page-by-page auditing and handling mismatched page counts
The comparison engine evaluates whichever page number is currently active in the viewer. If document A has 10 pages and document B has 12 pages, stepping to page 11 displays 'This page does not exist in this document' in the left panel while displaying page 11 in the right panel.
To inspect long documents effectively, scroll through pages with Sync Scroll enabled. If you identify unauthorized changes or need to adjust text formatting, import the file into Edit PDF or reorganize pages with Organize PDF.
Frequently asked questions
How do I compare two PDF files side by side for free?
Upload your original document into the left panel and the revised document into the right panel of TryDocsy Compare PDF. Use synchronized scrolling to step through pages simultaneously, and click 'Highlight Differences' to view visual variations marked in red, without requiring an Adobe Acrobat Pro subscription.
Why does PDF compare show an entire paragraph in red when only one word was edited?
TryDocsy compares documents visually pixel-by-pixel rather than analyzing text grammar. If editing a word causes sentences to wrap and shift downward, the pixels across the subsequent lines no longer match the original coordinates, highlighting the shifted block in red.
Can this tool tell me the exact text words that were replaced?
No. The tool computes a visual canvas diff rather than a semantic text diff. It visually highlights where changes occurred. To correct text discrepancies or add annotations, use /pdf/edit.
Are my documents uploaded to a server for comparison?
No. Both PDF files are rendered and compared locally in your web browser using pdf.js. No file data is transmitted across the internet.
What happens if one PDF has more pages than the other?
You can navigate through all pages. When you exceed the page count of the shorter document, the tool displays 'This page does not exist in this document' on that side while continuing to display the longer document.
What is the maximum PDF file size for comparison?
Each panel accepts a single PDF document up to 100 MB. File validation is performed instantly in your browser tab.