I can’t say for sure, I cannot get it to pop that same error again. I will try opening it from another iPhone with lockdown enabled later.

I wouldn’t even know where to start in looking at that PDF for some sort of iOS exploit payload, my guess is it would be extremely difficult to find (if it exists).

For the historical record, the Usenix Security paper PDF comment currently has 28 upvotes.

There is also your non-reproducible report that the PDF was incorrectly loaded as an Apple Wallet pass, which would require a web server MIME type of:

  application/vnd.apple.pkpass