https://draftable.com/compare is by far the best solution I've found for this, and it's a shame it's not more widely known about. It's not open-source, and their offline app is Windows only, but its ability to handle multi-page relayouts is far and above Acrobat's diff functionality (as the OP laments), and there's a free online version that's reasonably secure so long as you don't share the secret URL around. I've used it many times to obtain readable redlines when only given successive "baked" versions of a document, and it's a really useful tool for any B2B startup founder.
There's a free and opensource program called `diffpdf` that can compare both visually and by text. It has a GUI and works great, though it doesn't specially handle layout changes. It's included in the normal package sources in most Linux distros:
It's logically the same issue as with signing documents. You have to decide what aspect of the document you want to certify and ignore the rest. If you sign something in a complex document format you don't even know exactly what you are signing. Much of what you are signing is not even visible.
Things like legal documents should be restricted to plain text... and stuff like line endings should be standardized for this purpose.
Second that. This is THE solution for comparing any two PDFs (image or not). I’ve been using it for years almost on a daily basis. Part of its use is certainly derived from the excellent OCR engine it relies on. Also, this runs fully local, which is critical for legal purposes. (edit for context: I still use v14)
For visually comparing PDFs instead of textually comparing them, I use https://parepdf.com I work in publishing, so comparing printer proof PDFs is something we do regularly.
A guy I work with did his PDF diffing (basically testing whether invoices generated by new code match those generated by old code) by running the two invoices through ImageMagick and subtracting them from each other, basically looking for different pixels (maybe > $threshold) and then looking at the visual diff with missing pixels brightly colored. I thought that was really elegant.
BeyondCompare from Scooter Software [1] does a good job of comparing PDFs, although it does only compare the extracted text. But that is just one of the many many things it can do, so $60 (for the pro version or $30 for standard) gets you a lot more than just PDF comparison.
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Things like legal documents should be restricted to plain text... and stuff like line endings should be standardized for this purpose.
I don't see that happening! ;)
[1] https://www.scootersoftware.com