When I migrated the library between the two workstations, every entry was there, but most of the pdf documents in the entries were gone. The documents were actually there in the storage.
If you mean that you're getting a file-not-found error, the files aren't there. See Files Not Syncing.
You're just at your storage quota, so additional files weren't uploaded from the other computer. You'll need to add a storage subscription or transfer the data directory manually.
Sorry, I was lazy and concealed the actual process of the incident. In fact, this incident happened before and after I reinstalled the system. Before reinstalling the system, I exported and saved the document library. After reinstalling the system and reinstalling the software, this problem occurred after importing the previous document library.
An export isn't a backup. (If you lost files, you still did something wrong even in making the export, but that's beyond what we can help with at this point.) You should never wipe a computer without a full backup of the disk.
(And if you did use export and import, you would end up with duplicates of your entire library, since you're also using syncing. But files would still be missing on the original items, because you're at your storage quota.)
Upgrade Storage
You're just at your storage quota, so additional files weren't uploaded from the other computer. You'll need to add a storage subscription or transfer the data directory manually.