If you register for your account with your Harvard email address you receive free unlimited storage from Zotero. This means:
Backing up your Zotero Library to an external hard drive or other backup software ensures you won't lose data if something happens to your computer. Zotero developers don't recommend relying solely on a synced version to the Zotero cloud, as the data could be lost via automatic syncing.
When you install Zotero, a folder is created to store metadata, notes, and attachments locally on your computer. To find your Zotero file storage folder:
If you already have a good file management setup on your computer and would like to manage your files outside of Zotero, go the General section of Zotero settings and uncheck Automatically attach associated PDFs and other files when saving items (under "File Handling").
If you uncheck this, as you conduct your research, you will be required to download and manage all PDFs and other files on your own.
One way that you can manually associate files to Zotero items is by using the option of linking to a file.