The Compare Button
When the Compare button is pressed, DiffPDF does a high-speed scan of every pair of pages (~100 pairs of pages per second on the author's machine). To make the scan as fast as possible DiffPDF does a very rough check of each pair of pages—so it is possible that it identifies some false positives (i.e., page pairs that are really the same). False positives are quite rare. (There are no false negatives—differences are never missed.)