The characters are half on top of each other and make it unreadable. I think it has something to do with a specific font but I am not sure.
You probably need to give more details. PDF's you are viewing or creating ?
What program(s) are you using ?
Pdfs I view and create, i use zarutha as my pdf viewer but it also happens when I open the pdfs in Firefox. If i transfer the pdf to my phone then it works fine
Still short on detail. What desktop do you use etc.
Anyway my guess would be something up with your fonts as installed. Either a lack of them or something wrong with their installation.
So the PDF requests a certain font family but it can't be supplied.
It's just a guess.
Search on the Arch wiki 'fonts' and have a read.
But maybe try installing one of the packages here from Bitstream Vera ...... Noto fonts