That’s because they were hacked or exploited for an uncontrolled ability to upload searchable documents that get indexed. Very common with universities and gov sites. A real plague honestly.
And there’s a non-zero chance they tried hacking you by directing you to a malware payload.
I would never follow links found in a pdf. Heck I would never open a suspicious pdf like that given it could be the payload.
Use dbzero’s piracy wiki megathread of links to find reasonably safe sources for this stuff not random links.