Normally, I separate the download filename (what the server / person chose to call the file) from my own organization system file name.

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.

I only bother with files I expect to want to look at again after the next 48 hrs. Which is maybe 5% of what I download. Those usually also get moved from the downloads folder and filed somewhere useful. Or for some stuff like family snapshots they get categorized in a folder and not renamed.