What is the Compare PDF tool?
Two files with almost the same name sit in your downloads folder. One is the version you read last week, the other came back from the other side this morning, and nobody wrote a change log. This tool puts them on top of each other so the differences light up. You can compare PDF files online free here — drop the old document in as A, the new one as B, and read the result.
There are two layers to the comparison. The visual side renders both documents with pdf.js and works on pixels: it can tint every pixel that moved, show the pages next to each other, or fade B over A like tracing paper. The text side pulls the actual text layer out of each page and runs a word-by-word diff, so you get a report that says which words were removed and which were added, not just where the ink moved.
Be clear about what that is and is not. This is a mechanical PDF diff tool running in your browser — it finds differences, it does not judge them. It will not tell you that a changed payment term matters more than a reformatted heading, and it has no idea what your contract means. Reading and deciding is still your job; this just makes sure nothing slips past you unnoticed.
How to find differences between two PDFs
- Drop the earlier document onto Original (A) and the newer one onto Changed (B). The preview appears as soon as both are loaded.
- Pick a view from the Preview menu: Difference paints changed pixels pink, Side by side puts A left and B right, Onion skin fades B over A, and Text changes boxes the words that only exist in one file.
- In Difference mode, drag Sensitivity down to catch faint changes or up to ignore anti-aliasing noise. In Onion skin mode, use B opacity instead.
- Press Find changed pages to sweep the whole document. The strip under the preview turns green for identical pages and red for changed ones — click a number to jump there.
- Open Text comparison options if you want to ignore case, spacing or punctuation, then press Build full report for the word-level diff of every page.
- Read the report: struck-through red text was removed from A, green text was added in B, and each page carries a +added / −removed count.
Key features
- Four comparison views — pixel difference, side by side, onion skin and text changes, all switchable without reloading the files.
- Adjustable sensitivity — a threshold from 10 to 200 decides how big a colour change counts as a change, which is what separates real edits from rendering noise.
- Per-page change strip — one cheap low-resolution pass marks every page green or red, so a 90-page bundle tells you in seconds which three pages to read.
- Word-level text report — a longest-common-subsequence diff of the extracted text, with removals and additions marked inline and counted per page.
- Ignore switches — drop case, spacing or punctuation differences so a reflowed paragraph does not drown out the one clause that actually changed.
- Page-count mismatch handling — if B has pages A does not, the preview says so instead of silently comparing the wrong pair.
- Live percentages — the difference view reports what share of the pixels changed on the page you are looking at.
Benefits of using this PDF diff tool
- Confidential drafts stay confidential. Both documents are opened and rendered locally, so an unsigned agreement is never handed to a comparison service.
- Nothing to install. No desktop diff licence, no plugin, no account — the page works on a borrowed laptop as well as your own.
- Visual and textual in one place. A moved logo shows up in the pixel view, a swapped number shows up in the text report; you do not need two separate tools.
- Fast triage on long documents. The change strip narrows a big file down to the pages worth reading before you spend attention on them.
- No watermark and no page cap. The tool draws on a canvas in your tab, so there is nothing to stamp and no quota to hit.
Who should use this tool?
Lawyers and contract managers
When a redraft comes back without tracked changes, the text report is the fastest way to see which clauses moved. Turn on Ignore spacing changes so a reflowed paragraph does not look like a rewrite, then read the pages the strip marked red. Treat it as a first pass that tells you where to look, not as legal review.
Editors and proofreaders
Comparing a proof against the previous round catches the corrections that were missed and the ones that were applied twice. Onion skin is good for layout drift — a caption that slid down a line shows up immediately when B is faded over A.
Developers and report builders
If your code generates PDFs, this is a quick regression check: render yesterday's output as A and today's as B, run Find changed pages, and confirm that only the pages you touched differ. The pixel percentage per page is a useful sanity number.
Finance and procurement teams
Two revisions of a quote or an invoice schedule can differ by one figure buried in a table. The word-level diff surfaces that figure instead of leaving you to compare columns by eye.
Frequently asked questions
Is comparing two PDFs here really free?
Yes, with no signup and no page limit. Both files are rendered in your own browser, so there is no processing bill behind the tool and nothing to charge for.
Are my documents uploaded anywhere?
No. The PDFs are read by JavaScript in your tab and drawn to a canvas on your device. Once the page has loaded you can go offline and the comparison still works.
Why do two scanned pages show differences when the words are identical?
Because scans are images. Even a reprint of the same page differs in brightness, alignment and speckle, so pixel comparison lights up everywhere. Raise the sensitivity to calm it down, and for a real word-by-word answer run both files through the OCR tool first to give them a text layer.
Can it compare two document versions that have different page counts?
Yes. Pages are matched by number, and where a page exists in only one file the preview says so and the report treats the missing side as empty. Big insertions early in a document will shift everything after them, so expect the pages beyond the insertion to be flagged.
Does the text report work on any PDF?
Only where there is a text layer to extract. A page that is a photograph of paper has no text, and the report says so and points you at the visual preview instead. Run OCR first if you need the words.
What is the difference between the four preview modes?
Difference tints every changed pixel pink over a faded grey page. Side by side shrinks A and B into two panes for direct reading. Onion skin lays a semi-transparent B over A to expose small shifts. Text changes ignores appearance and boxes the words present in only one file.
Can I check contract changes in a password-protected PDF?
Not while it is encrypted — the file fails to open and you will see an error. Remove the password with the Unlock PDF tool first, then compare the unlocked copies.
Does side by side PDF comparison work on a phone?
It does, though the two panes get small. On a narrow screen the difference view or the text report is usually easier to read, and heavy documents render faster on a laptop.
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