Years ago I standardized on a pretty flat folder hierarchy for such PDFs as well as a file naming convention such as the following examples: topic-author-YYYY-MM-DD.pdf or article-title-website-domain-YYY-MM-DD.pdf, etc. In about 2 or 3 times in my life, i have also drafted a small summary of the content of a PDF into a sibling README (text file) for ease of findability later on. This was because those few PDFs were a very old docs that were scanned in, etc. Also, the sibling README file would be named the exact same as the PDF, though have a file extension of .txt or .md or ...-README.txt, etc.
Nowadays, there are more tools available - as others have cited - which help with finding metadata. But i like to keep things simple, and because there's still way too many PDFs whose metadata starts with "Microsoft Word...", that i still stick to my file naming convention to help give hints about the contents.