I know Calibre can remove DRM, but it seems that Calibre does not remove things like watermarks, references to the buyer by name, etc. Now maybe I can try to find those manually, but that is an error prone process. Plus, what if they embed a unique digital signature that ties back to me? I understand that this is a very uncommon practice, but I do not want to find myself in a bad place.
I suppose the only way to remove a digital signature of any sort is to buy two of the same e-book by different people, diff them, and remove anything that differentiates them.
Is there any tool that does this or automates the process? am I being too paranoid, and this is not a real threat?
gImageReader or ocrmypdf will get you the pdf text, but after the text will need fiddling with and cleaning. Use LibreOffice, languagetool, write-good, etc to make finding the oddballs easy.
pdftk is what you want for editing pdf metadata.
Gimp is what you’ll need for editing images, Looking for watermarks, smoothing edges, lowering quality, introducing random noise, etc.
exiftool is what you’ll need for image metadata. Or take a screenshot, add a bit of noise or de-noise, and add back to the new pdf.
Scrivener or LibreOffice if you want to polish/republish, though that’s a ton of work.