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.
ALT [A list of metadata for a PDF, the important field being two "Unknown:<long random character string>" fields that are color coded to indicate that they have been changed between versions.