Comment by donaldihunter

I came here for this. E.g. https://www.alchemistowl.org/pocorgtfo/pocorgtfo19.pdf

"This file, pocorgtfo19.pdf, is valid as a PDF document, a ZIP archive, and a HTML page. It is also available as a Windows PE executable, a PNG image and an MP4 video, all of which have the same MD5 as this PDF."