I’m writing this half as a filler post and half because I’ve met a lot of people who are using bad ways to organize their pdfs.
For academics or anyone who needs to organize papers, I would strongly recommend using Mendeley. It’s basically a booru for pdfs, and it has good automatic data entry as well. You can upload and access your papers, it automatically scrapes the citation info and generally works well. By adding articles to it every time you read a new one, it lets you search much more effectively for claims that you remember someone made but have no idea where they came from. You can also organize ebooks this way.
A worry that discourages me from any cloud product is that there may be no way to export data if it isn’t supported any more. I’m taking that risk in this case.