I have the same issue. I thought the point of cloud storage was to have the .pdf available from any machine...but mine are only available on the machine that I used originally to find the document. On other machines, I get "It may have been moved or deleted outside of Zotero, or, if the file was added on another computer, it may not yet have been synced to or from zotero.org."

How can I force-sync uploads to the Zotero cloud?