What changed between two PDFs?
Drop a draft and its revision — or a preprint and the publisher's proof — and see every difference: pages aligned side by side, an overlay, or a clean text diff that survives re-typesetting. Everything runs in this tab; nothing is uploaded.
drop both files at once anywhere on this page — or
pages aligned, not just numbered
An inserted cover page or a re-flowed chapter doesn't make every later page look "changed": pages are matched by their content, and pages that were merged or split are shown with their counterpart.
a text diff that survives typesetting
Running heads, page numbers, hyphenation and font changes are ignored; the text of both files is diffed as one document, word by word, with relocated passages shown as moves rather than edits.
local by design
A strict Content-Security-Policy forbids all network requests, so the page couldn't upload your files even if it wanted to. Load it once and it works offline. Open source, AGPL-3.0.