Zotero

Creating an account on the Zotero website allows you to keep a copy of your library on the Zotero server, which is very handy if you do not always use the same computer or if you want to work collaboratively. 

After creating your account, enter your username and password in your Zotero Settings: Sync., enter your username and password. You will need to repeat this process on each of your computers. Any change made to your database will be automatically synchronised.

You can synchronise an unlimited number of references, notes, and collections. PDFs and other attachments take up storage space, which is limited to 300 MB for free. You can purchase additional storage space if necessary.

If you want to save money you can:

• store your files on an external server: create an account with a WebDav service. Go to the Sync tab of the Zotero Settings menu, and in the File syncing section, select WebDav from the drop-down menu after "Sync attachment files in My Library using". Enter the address of the WebDav server you chose, your username and your password, and click on Verify Server.
• if you want to store your files on Google Drive or Dropbox, you can use the Attanger or the ZotMoov plugins. Both allow you to give Zotero the path to the external directory where your PDFs are stored.
  How to proceed:
  - with Attanger
  - with ZotMoov

Both of these options can only be used for the attached files in your personal library, not for group libraries.