Your exchanging pdf files with an editor for a journal publication. Each time you send a list of modifications to be applied and the modified pdf file comes back to you, usually with some additional modifications that you didn’t ask for. How to spot these change easily and to make sure they are relevant?
The solution that come to mind is to use a pdf diff tool, similar to what you usually use for text files or source code: diff, diff3, kdiff3, meld are such examples. However, a quick search on the internet couldn’t reveal any tool fulfilling the conditions (being free and open source are mandatory conditions).
The easy trick is to first convert these pdf to text file and then use the usual text comparison programs:
pdftotext file1.pdf pdftotext file2.pdf kdiff3 file1.txt file2.txt
and that’s it, you are now able to see what has been modified between the two files. Don’t count on that to be able to make a merge though!