03 / WHAT COMPARE PDFS
What Compare PDFs does
Compare PDFs examines two PDF files page by page, in order, and reports differences in both the text and the visual appearance. For each pair of pages, you get a verdict: unchanged, text-changed, visual-changed, or changed. A page that exists in only one document is marked added or removed.
The output is a ZIP containing two files: comparison.pdf, the visual report, and report.json, the same findings as structured data. The PDF report shows one page per compared pair, with a dark header band naming the page and its verdict. Below the header, the image highlights the regions that differ. The header also prints how many words were added, how many removed, and what percentage of pixels differ.
The JSON report gives you the same totals and a per-page breakdown, which is useful if you need to feed the results into another system.
04 / HOW TO COMPARE TWO
How to compare two PDFs
To start, select the two PDF files you want to compare. They are uploaded together, and the job runs on the server. You can also adjust the resolution using the DPI field. The acceptable range is 72 to 200 DPI, with a default of 110. Higher DPI captures finer visual details but may take longer to process.
The sensitivity of the pixel comparison is fixed in the engine and is not adjustable. You only control the two files and the DPI.
Each job compares exactly two files. The files may be up to 50 MB and 200 pages each, but if the longer document exceeds 200 pages, the comparison is refused.
05 / WHAT HAPPENS TO YO
What happens to your files
When you upload a PDF, it is first scanned for malware before any processing begins. The file type is verified by reading the actual bytes, not by the file name, so renaming a non-PDF to .pdf will not get it through. If the file is not what the tool expects, it is rejected before any work starts.
After processing, the produced files are re-opened and parsed to ensure they are valid. If something went wrong and the output is broken, the job fails instead of handing you a corrupt file.
Uploaded files are stored in private object storage. For anonymous uploads, they are deleted after 1 hour. If you are signed in, they are deleted after 7 days.
06 / IMPORTANT LIMITATI
Important limitations
This tool compares documents page by page in order. It does not detect that a page was inserted and shift the remaining pages. If you add a page early in one document, every page after it is likely to be reported as changed, even if the content is identical but moved.
The visual comparison is based on rendered images at the DPI you choose. Text that is rearranged but looks the same will be marked text-changed. Visual changes such as a shift in layout or an image alteration are caught when they affect the pixels.
This tool does not merge the two documents into a single annotated copy. It produces a side-by-side-style report, one page per comparison.
07 / INTERPRETING THE R
Interpreting the results
Pages are labeled with one of six verdicts: unchanged, text-changed (wording differs but the visual layout looks the same), visual-changed (it looks different, but the words are identical), changed (both text and visuals differ), added, or removed.
Use the percentage of differing pixels to gauge the visual impact, and the added/removed word counts to understand textual edits. For a deeper analysis, the JSON file contains the same data in a machine-readable form.
If a page is marked visual-changed, check the highlighted regions on the PDF to see exactly what shifted or altered.
FAQ
Common questions
How do I compare more than two PDF files?
This tool is designed for exactly two files at a time. To compare multiple versions, run separate comparisons for each pair or use another approach.
Why does my PDF comparison show all pages as changed?
Because the comparison works page by page in order, inserting one page early in a document shifts every subsequent page. So even if the text is identical, all later pages will appear changed. Try comparing documents that have the same page order.
What file size and page limits apply?
Each PDF can be up to 50 MB and up to 200 pages. If the longer document exceeds 200 pages, the comparison is refused.
Can I adjust the sensitivity of the visual comparison?
No, the sensitivity is fixed in the engine. You can only set the DPI resolution, which affects the detail level of the visual scan.
Is my uploaded PDF kept private?
Files are stored in private object storage. For anonymous uploads, they are deleted after 1 hour. For signed-in accounts, they are deleted after 7 days.
Why is my PDF rejected even though it has a .pdf extension?
The tool checks the actual content of the file, not just its name. If the bytes do not match a valid PDF, it is rejected before processing.