How the comparison works
The text of each page is read from both PDFs and lined up in reading order. Page 1 is compared with page 1, page 2 with page 2, and so on, and the words on each pair are matched to find the smallest set of additions and removals. Pages that exist in only one file are shown as wholly added or removed.
What it can and can't see
The comparison covers the words in the PDF. Changes to pictures, colours or layout that don't change the text won't show up, and scanned pages need text recognition first, because a scan is a picture of words rather than text. If a page was inserted near the start, later pages shift by one, so expect the following pages to differ too.
Private by design
Contracts and drafts often hold sensitive details. Both files are read and compared inside your browser and are never uploaded, so they stay on this device.