The forensic compare: exactly what changed between two PDFs
Every difference, and nothing that is not one
Choose the earlier version and the later one. Both are drawn at 300 dots per inch by the renderer this service checks its own corrections with, and a pixel counts as changed when it differs at all: the tolerance is 0. The same page drawn twice is the same pixels, so a file saved again by another program shows no difference, and a comma that became a full stop does.
Where it changed, and which words
Changed pixels are gathered into places about a line of text apart, and numbered. Step from one to the next, with the two pages side by side or one laid over the other, on a computer or a phone. Beside each place are the words that changed, struck through and underlined with the words round them, read from the page's text or, on a scanned page, read where the page changed. A page added or taken out is shown as that, and the pages after it still line up with their partners. Form fields, comments and links are compared too, by what they hold, because a reader does not draw them onto the page.
The report
With a subscription or a pack, download a report of every change — both files' fingerprints, how the comparison was made, a map of each changed page and the words as a redline — and a spreadsheet of the words that changed, whose cells can never run a formula. The report shows outlines and words, not your pages, so it can be passed on without passing on the documents.
What it cannot see
Two scans of the same sheet of paper differ everywhere, because the scanner does: this compares files, not paper. A document set again by another program shows its letters as moved, and its words say that nothing was reworded. Files of up to 50 pages are compared, at the size your plan uploads.
Compare two PDFs now — it opens the tool, with nothing to install.
The record
Comparing changes neither file. What was compared, and how many changes were found, is in your account's trail; the words themselves are kept with the comparison, and deleted with it or with either file. Files are deleted after 90 days, or whenever you erase them.