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Comparing Scanned PDFs vs. Text PDFs: What's Different
Not every PDF has real text to compare. Here’s exactly what happens when Compare PDF encounters a scanned or image-only page, and why the result is still useful.
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Why scanned pages can't get a text diff
A scanned or photographed PDF page is really just a picture of a document — there’s no underlying text to extract at all, so there’s nothing for a line-by-line text comparison to work from.
How the tool tells the two apart
Each page is checked for how much real, extractable text it actually contains. A page with little to no extractable text is treated as scanned; a page with a substantial amount of real text is treated as a text page. This is decided per page, not per document, so a PDF that mixes typed pages with a scanned signature page is handled correctly.
What you still get for scanned pages
Scanned pages fall back to a visual comparison of the rendered page image — still matched by content (not just position) across the two files, and still reported as unchanged, changed, added, or removed. You lose the line-level "what changed" detail, but you still get accurate page-level change detection.
Mixed documents
In a document with both text and scanned pages, each page uses whichever signal actually applies to it — text pages get the full text-and-visual check, scanned pages get the visual-only check — rather than the whole document falling back to the weaker signal just because one page is scanned.
Comparing Scanned vs. Text PDFs FAQ
- Will comparing two scanned PDFs work at all?
- Yes — they’re compared visually instead of by text, and you still get accurate Changed/added/removed results per page.
- Can I get a line-level text diff for a scanned page?
- No — there’s no extractable text on a scanned page for a line-level diff to work from; the visual comparison is the available signal.
- How does the tool decide a page is "scanned" versus "text"?
- By how much real, extractable text that specific page actually contains — decided independently for every page, not for the whole document at once.
- What if one PDF is scanned and the other has real text?
- Each page pair still gets matched by whatever signal applies — a scanned page is compared visually even against a text-page counterpart.
- Does a scanned page ever get a false "Changed" from rendering noise alone?
- The visual comparison uses a tolerance for small rendering/anti-aliasing differences so trivial noise alone shouldn’t trigger a false Changed result.
Related guides
- How to Compare Two PDFsUpload an original and a revised PDF to see exactly which pages changed, which were added or removed, and which lines of text were edited, all processed locally in your browser.
- How PDF Comparison Detects ChangesThe real mechanism behind Compare PDF: how pages are matched by content rather than position so an inserted page doesn't cascade false changes, and how text and visual differences are checked independently.
