As far as I can tell, Notability doesn't really sync with Dropbox. It provides a one-way backup of notes to DropBox but uses iCloud for actual sync. Also, when I try to do DropBox backup on two iOS devices, I get a warning that it can be used on only one. So, I think what you are trying to do isn't possible. You probably have to add the files back manually. Are your files in iCloud, as well? If so, that would be where you go for restoring. I'd contact the developer about your problem.