My recommendation for research papers would be Zotero. You have the option of using their cloud (free and paid tiers are available) or you can simply point it at where you’d like to store your documents (this will be up to personal preference, I’ve seen it done using sync tools and self-hosted “clouds” such as Nextcloud).

I’ve personally been using Zotero for just over 6 or 7 years and it’s honestly been such a game changer for my productivity. Not only does it help keep track of my research, but it also has a very robust plugin ecosystem (such as automatically fetching missing DOIs, more advanced file renaming, fetching stats on papers such as how frequently it’s been cited, etc.).