While nearly everything is on the internet, lots of things (old books, technical papers) are written with PDF. The main problem is that I’m constantly moving back and forth between different devices, so what is the best solution for that problem.
Well if you have Apple devices, the answer is pretty easy, the iOS Books application actually can sync those PDFs on iCloud, so when you save a PDF to your phone, it will automagically appear in your MacBook. You just have to make sure to sign up with the same Apple ID on both devices.
The main gotcha is that when you do this, it is sometimes hard to find the PDF on other devices. The problem is that they don’t have a sort for “most recently added” just for creation date. So you have to search for the title to figure out where it is.