The question is never "are these different"
You already know they are different. Somebody sent the contract back "with a few small amendments" and did not say which. The question is what moved, and reading both versions side by side to find out is an afternoon.
So this marks up the wording in place: what was taken out struck through, what was put in highlighted, in the order it appears.
How this tool works
The pages are lined up first. This is the part that decides whether a comparison is useful or worthless, and it is where the free tools fall down.
Insert one paragraph on page two and everything after it shifts. A tool that compares page one to page one and page two to page two then reports every remaining page as completely rewritten, which is true in the narrowest sense and no use to anybody.
Instead the pages are matched by how alike they are, so a page is still recognised as itself after a sentence changes, and an inserted or deleted page is reported as exactly that. The threshold is deliberately strict, because two unrelated pages of the same contract already share a great deal of language: the, shall, contractor, section. Matching too readily pairs a deleted page with the one that took its place and invents a rewrite that never happened.
Then the wording is compared within each pair, word by word, and runs of changes are merged so a rewritten sentence reads as one block rather than forty fragments. Unchanged pages are skipped entirely, which is most of the work on a long document that barely moved.
It tells you when nothing changed
The most common real answer, and the one worth stating rather than implying with an empty list. If the wording is identical page for page, it says so.
What it does not compare
Only the words. Two documents can read identically and differ in layout, fonts, images, table borders or colour, and this will call them the same. If a drawing has been revised, that is a different job: overlaying two revisions and seeing what moved.
Scans have nothing to compare. A scanned PDF holds a picture of words rather than words, so there is no text in it. The tool says so rather than reporting two identical empty documents. Run both through recognition first, and treat small differences with suspicion: they may be a recognition mistake rather than an amendment.
Order matters. The first document is what the second is measured against, so putting them the wrong way round turns every addition into a deletion. Nothing is damaged, you just read it backwards.
Two versions, and one of them is theirs
Both files are read and compared on this page using your own machine. That matters here as much as anywhere on the site: the two documents are almost always a contract and the version somebody has sent back, and the usual free option is to upload both to a company you have never dealt with.
There is no page limit and no file size limit, because there is no server being paid for by the page.
Common questions
How do I compare two PDFs for free?
Drop in the older version and the newer one and press compare. You get every wording change, marked up in place, with a note of which pages were added or removed. No account and nothing uploaded.
What happens if a page was inserted?
It is spotted and reported as a new page, and everything after it still lines up. That matters more than it sounds: without it, one inserted page makes every later page read as completely rewritten, which is what most free comparison tools do.
Does it compare drawings or just words?
Words. Two documents can read identically and still differ in layout, fonts or drawings, and this will call those the same. For drawings, overlaying two revisions is a different job.
Can it compare scanned documents?
Not directly. A scan holds no text, so there is nothing to compare word by word. Run both through the recognition tool first and compare the results, remembering that any difference might be a recognition mistake rather than a real change.
Are either of the PDFs uploaded?
No. Both documents are read and compared on this page using your own machine, which matters when the two files are usually a contract and the version somebody sent back with amendments.