More fun publisher surveillance:
Elsevier embeds a hash in the PDF metadata that is *unique for each time a PDF is downloaded*, this is a diff between metadata from two of the same paper. Combined with access timestamps, they can uniquely identify the source of any shared PDFs.
You can see for yourself using exiftool.
To remove all of the top-level metadata, you can use exiftool and qpdf:
exiftool -all:all= <path.pdf> -o <output1.pdf>
qpdf --linearize <output1.pdf> <output2.pdf>
To remove *all* metadata, you can use dangerzone or mat2
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@f0x
I suspect it should be a good idea to compare two PDF from two different source. If the hash match, it's all good. If the it doesn't, strip the EXIF. If it still doesn't match... find the difference somehow.
@jonny