>>882687 (OP)
>Many can't even open PDF or music files from my SD card
I think it has something to do with Google fucking up directory permissions in recent Android versions.
Instead of just opening files form directly via Linux syscalls you need to do it through Android special API. I am not sure on the details, but there're other directory structure-related and general conceptual purpose of sdcard changes too, that make things hard.
Not sure what problem they were solving exactly. Some conspiratards say it's a push to use more Cloud(tm), while Google claims it's for extra security.
In practice, though, as usual, this resulted in bad actors with incentive to jump over all the hoops, while honest actors are struggling to keep up.