A reference appearing on a second device does not prove its PDF is available there. Zotero's sync documentation distinguishes library-data synchronization from attached-file synchronization.
For the Zotero alternatives page, test a citation, a note and an attachment on two devices. Then open the attachment on the second device rather than checking only the item title. Follow Zotero's supported storage guidance; putting the live data directory in a generic cloud-sync folder can risk database corruption.
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