If the program is closed, items will be saved directly to the Zotero server, which is limited to saving in the base of My Library (can't specify a collection or group). Saving to the server will also always download the PDF and page snapshot if available (it doesn't have a way to detect the settings set in the Zotero program).
On Mac, you can close the Zotero window without exiting the program if you would like to keep the program open in the background.
Upgrade Storage
On Mac, you can close the Zotero window without exiting the program if you would like to keep the program open in the background.