Compare PDF lets you open two versions of the same document and inspect them page by page. You can view the original and revised pages side by side, or turn on difference highlighting to paint changed pixels red and see a similarity score for the current page. Both files are processed in your browser and are never uploaded.
How to compare two PDFs
- Drop the earlier version into Original PDF and the newer version into Revised PDF.
- Use Previous and Next, or type a page number; the range follows the longer file.
- Compare side by side on a computer; on a phone they stack.
- Keep Highlight differences on to see changed pixels in red, with a similarity percentage for that page.
- Click Compare all pages to score every page at low resolution. You can stop the scan and keep the results collected so far.
- Use Download diff image to save the current page's difference overlay as a PNG.
Side-by-side viewing versus difference highlighting
The side-by-side view is the place to start. Each page is rendered separately, so you see the layout, fonts and images from each file. This helps when judging design revisions or checking a signature block.
Difference highlighting is mechanical. Both pages are scaled onto one canvas and each pixel's colour is compared. Pixels that differ beyond a small tolerance turn red; unchanged areas fade grey. The similarity percentage is the share that matched. A value of 100% means the rendered pages looked the same at that resolution, not that the documents are legally or textually identical.
Side by side shows what the page looks like; the overlay shows where the output changed. Use both to find small changes.
What a visual diff can and cannot tell you
This is not a text comparison. The tool does not extract words or produce tracked changes. If a sentence is reworded but occupies the same space, the diff may show only the pixels around the altered letters. If a paragraph moves to another page, the result may be a large red area rather than a note saying that text moved. For clause-level review, use a word processor's comparison feature and a careful read-through.
Rendering can create differences that are not meaningful edits. A font substitution, a different export setting or changed anti-aliasing can alter many edge pixels even when the layout looks almost the same. A shifted margin, page size or rotation changes alignment and can make unchanged content appear red. The tool does not align pages or compensate for skew.
Scanned documents are the hardest case. Two scans of the same page rarely have identical noise, brightness or texture, so the score can be low even when the text has not changed. A scan compared with a digitally generated PDF will look different across most of the page. Visual comparison works best when both files come from the same source with the same page size and fonts.
There is also a resolution limit. The current-page diff uses a modest canvas, and the full scan uses a smaller one for speed. Small marks, thin punctuation and faint watermarks may be missed or blended into nearby pixels. Treat the percentage as a guide for where to look, not as proof that nothing changed.
Different page counts and page sizes
The viewer follows the longer file. If the original has 10 pages and the revised has 12, you can move through pages 1 to 12. On pages 11 and 12, the original panel says No page 11 in this file instead of showing a blank canvas. The table marks them as existing in only one file and excludes them from the compared count.
Different page sizes are handled by scaling both pages onto one comparison canvas. The diff is therefore visual rather than pixel-exact. A move from A4 to US Letter, or a change in orientation, alters the scaling and may produce red areas that reflect the size change as much as any edit.
Page numbers are compared by position, not by content matching. If a page was inserted early in the revised file, page 3 of the original may now be page 4, and comparing page 3 with page 3 will show unrelated content. In that situation, scan the table to find the shift, then step through manually.
When comparing PDFs is useful
Visual comparison is handy for contract revisions where you want to see whether a clause, date or signature block moved; for design proofs where a logo, colour or spacing changed between rounds; and for reports where a chart or table was regenerated. It also helps check a PDF export after a template or font update.
It is less suitable as the only check for legal redlines, accessibility or text accuracy. Use it to find pages worth attention, then review them in a PDF reader or text comparison tool. Keep the original files, because this tool does not modify or save either PDF.
Privacy: your files never leave your browser
The PDFs are opened, rendered and compared by JavaScript running in your browser. They are never uploaded, and there is no account, file history or cloud copy. Closing the tab releases the documents. Large files still need enough memory, so a long or image-heavy PDF may be slower on a phone.
Related PDF tools
- To combine two versions into one file, use Merge PDF.
- To separate a long document into smaller parts, use Split PDF.
- To reorder, rotate or delete pages before comparing, use Organize PDF.
- To add page numbers for easier reference, use Add Page Numbers to PDF.
Compare PDF FAQ
Is Compare PDF free to use?
Yes. You can compare as many PDFs as you like without signing up, and there is no watermark. Practical limits come from your browser's memory and rendering time.
Are my PDFs uploaded anywhere?
No. Both files are read and rendered in your browser using PDF.js. They are not sent to a server or stored in an account.
Does this show text changes like track changes?
No. It compares rendered pages, not text. It can highlight where pixels changed, but it cannot list inserted or deleted words. For clause-by-clause review, use a word processor's comparison feature.
Why does a page look different when I only changed a little text?
A small edit can move nearby lines, change line breaks or alter anti-aliasing around the text. One word can produce a noticeable red patch. A low-contrast change may produce only a faint mark.
Why is the similarity score high when the page looks different?
The score is the percentage of comparison pixels that matched. Text covers a small fraction of a page, so a changed sentence can still leave most of the white background unchanged. Inspect the red overlay before relying on the number.
Can I compare scanned PDFs?
Yes, but scans are less reliable. Scanner noise, skew, brightness differences and paper texture can make unchanged pages look different. The tool does not deskew scans, so check faint text and stamps carefully.
What happens if the PDFs have different page counts?
You can still compare them. The viewer goes up to the longer file, and a missing page is shown as No page N in this file. The table marks pages that exist in only one file.
Can I download the full comparison report?
No. The only download is the current page's difference overlay as a PNG. It does not produce a combined report, a modified PDF or a file with both versions.