To sync your account from computer to computer and keep a backup of up to 300MB of files available in the cloud for free, follow these steps for Mac and PC.
Synced attachment files (i.e. article PDFs, etc.) in My Library using Zotero and downloading files as needed. If you use shared group libraries, we recommend the same settings. Zotero provides 300MB of free storage per user. If you need additional storage, you may purchase it or you may use one of the suggested WebDAV options.
Zotero Login & Sync for Mac
Open the Zotero application, then click Zotero > Preferences.