Is this for a linked PDF, or one in local Zotero storage ?
Usually that error message does tell you where Zotero *thinks* that file should be. When it doesn't, it can mean it's a linked PDF and that you have set a Linked Attachment Base Directory on one computer (which changes how the file's location is stored) but not on this computer. So Zotero has no information to find the file. https://www.zotero.org/support/kb/missing_linked_file
In any case, the solution often involves knowing where that file *actually* is, and comparing that to what information Zotero has to find it, like the LABD.
A (non-ideal) alternative that would tell you exactly where Zotero thinks the PDF should be is to wait for Zutilo to be updated for Zotero v7 (apparently in progress). Zutilo can provide the stored location information. I am not sure if there is another plugin that can do that. Or you can look in the sqlite database (itemAttachments table).
Upgrade Storage
Usually that error message does tell you where Zotero *thinks* that file should be. When it doesn't, it can mean it's a linked PDF and that you have set a Linked Attachment Base Directory on one computer (which changes how the file's location is stored) but not on this computer. So Zotero has no information to find the file.
https://www.zotero.org/support/kb/missing_linked_file
In any case, the solution often involves knowing where that file *actually* is, and comparing that to what information Zotero has to find it, like the LABD.
A (non-ideal) alternative that would tell you exactly where Zotero thinks the PDF should be is to wait for Zutilo to be updated for Zotero v7 (apparently in progress). Zutilo can provide the stored location information. I am not sure if there is another plugin that can do that. Or you can look in the sqlite database (itemAttachments table).