[jlduan]

It seems Zotero still doesn't support 3rd party cloud service (Dropbox, Google Drive, iCloud etc).

I really recommend Paperpile, especially its Chrome extension. In my opionion, nothing beats Google Docs on collaborative writing.

[d4rkp4ttern]

Agree on Paperpile as some others have commented. Dead simple to use. For annotating PDFs nothing beats handwriting, so for that I use iPad 12.9” plus GoodNotes app.

[BostonEnginerd]

I believe that they support WebDav.

https://www.zotero.org/support/kb/webdav_services

You also have an option to use the Zotfile extension to sync with other services.