0x17 is a PDF that's also a valid ZIP and valid firmware for Apollo Guidance Computer.
And that's just the tip of an iceberg. This project is absolutely crazy, in the most positive possible way.
Wait, what?
Edit: 0x15 has the additional feature that viewing it in Chrome makes my work machine throw a "Threat detected: A threat has been blocked and quarantined" message, so I think I'm going to avoid this site now. Not to say that I don't trust this random PDF hacker (...that sounded less sarcastic in my head), but I don't need IT asking me pointed questions about what sites I've been visiting.
PoC||GTFO is a well known project, and has a tradition of turning their PDFs into absurdly polyglot files. Your Chrome is giving you a false positive here.