If I go back to look for the file, it's likely to get lost so I'll probably just go hunt for the original PDF again online to download
Is this a problem to you or should filenames actually not contain anything about the subjects (like reflecting metadata)?
I guess my "solution" is manually requesting more changes of filenames from people until it catches on as a convention, unless someone convinces me that the random naming I see is fine for reasons I'm missing
So if I download or get sent "Book.pdf", I'll rename it to how I'll remember it -- "Book Title - Author.pdf", etc.
That being said, I don't think there's any right answer here, it's usually just a matter of time and energy. If I had to enrich every single file I download with a great title / detailed metadata / etc that I'd need to find that file later, that's all I'd do all day.
eg https://www.technobezz.com/how-to-set-firefox-to-always-ask-...