[Zubersoft]

I don't think using the sync library feature is the best option for the way in which you are working. It would be better to just use Import->Batch Import to pull in all your files, as there is an option to ignore duplicate files as well as an option to update existing songs if an updated file is found. So then you can just batch import the necessary folder to pull in any new versions of PDFs. I'd also recommend periodically creating a library backup file under Settings->Backup and Restore->Backup Library to safeguard your library, and you can store that in Dropbox separately.

Mike