Hi, does this only happen for a specific set of PDFs or all of them, regardless of source?
In any case, the said files might be getting corrupted by malware trying to infect the documents, eventually failing to infect them correctly. If it is the case I'll redirect your post to the Viruses/Security subforum in the hope someone will help you clean the computer.
Can you also try to open the files with different PDF reader, e.g. Firefox if it is installed (it can read PDFs directly thanks to
from human import idiocy