Well. Even files can be attached to pdfs, in few case normally as a "pdf source" in reproducible research... But well... We have pdftk and other simple utilitie

Well. Even files can be attached to pdfs, in few case normally as a "pdf source" in reproducible research... But well... We have pdftk and other simple utilities to manage pdfs metainformation... Also it's common to embed other kind of "steganographic" information that's maybe really hard to discover, the simpler are like "dot printers", simple white-on-white etc content in plain pdf that can be read easily by a bot, other may use single pdf's content with a Caesar-like cipher etc. Pdf are a vast and not-so-clean world...