Well, you could put most of the payload in other parts of the file. People probably wouldn't notice if they didn't go looking. And the small part could exploit a popular codec instead of being an executable itself.

I didn't say it wasn't, it was for context. Also you need to be polyglot + have enough space for the collision computation.

I just checked, it should be really easy. A piece/chunk size in torrents is 64kB. The total size of each PDF in the paper is 2.7kB.

A more common chunk size would be 256 kb - 2 MiB. Yeah, very doable.