Here are some instructions for recovering a Paperless library file. Before I list these directions I would need to recommend to you to regularly backup your files (if you don't already). Ok, without further adieu, here is how to possibly fix your Library file:
locate the file in the Finder
right click on the file and select "Show Package Contents"
Inside the Library Package, you'll see a few items:
For the time being, place the Documentwallet.documentwalletsql file on the desktop or somewhere other than this location.
Go into the Backups folder and copy the most recent backup and paste the file into that main package area.
Rename the file to from "DocumentWallet 1.documentwalletsql" to "DocumentWallet.documentwalletsql"
this is replacing the main database with a recent backup.
now, go out of the package and double-click on this Paperless Library. If it opens you should be golden. Please check to see it all your recent additions are there. if the database backup is old it is possible that there might be some recent activity not accounted for.
If the Library doesn't open "Paperless Library cannot be opened: File does not exist" the datable is still corrupted. You may try the other files in the Backup folder or try to restore from a Time Machine backup or another external backup.