I think GP is referring to the ReadCube Papers app, which is actually also available on Windows, mobile, and as a web app:

https://www.papersapp.com

However, in light of its monthly subscription pricing, I'll be sticking with Zotero which does everything I need on a desktop computer, and also with Linux support. Zotero being free and open source gives me confidence that my data won't be encrypted in a user-hostile way like Mendeley did to its users after it was acquired by Elsevier.