Zotero's sync feature can keep your library up to date on all your devices and make sure your sources and citations are backed up in the cloud. This requires you to have already set up a Zotero account. After that:

1. Open Zotero preferences (via the gear menu) and select the Sync tab.
2. Enter your Zotero username and password.
3. Check the "sync automatically" box.
4. Check both boxes under File Syncing and choose Zotero storage for My Library. This will sync your PDF attachments as well as citations (more info).
5. Click the green circular arrow button at the top right corner of the Zotero window.
6. Zotero will upload your library to the server.

Repeat this configuration on each of your devices. Any updates you make on one of your devices will be reflected on the others.

For more details and help troubleshooting sync problems, check the Zotero site.