Why are the names of PDFs always random letters and numbers or "full text"?

That’s just how whoever created the file named them. It’s in no way special to PDFs. Simply rename it as soon as you download it – or, if you like, rename it after you’ve clicked “save”.

Most likely they assign it a random name when they store it to avoid any naming conflict. Things can get messed easily with different languages and Unicode and spaces and special characters and duplicates and so on. It is easiest to just generate a known good name for the file.

And remember that something like www.whatever.edu/fu@gly///lulZ}{_#d>>punch.pdf is going to break if it needs to be a URL to download the file.

They probably have some kind of system in place that names it something illogical so that people cruising the internet searching specifically for free access to the paper cannot find it.

Is it possible that these file names are computer generated? (both to avoid naming conflicts and to make them hard to find).