Accessing a PDF stored in iBooks (on a jailbroken device)
There are various files (.plists residing in /var/mobile/Media/Books), but the only one that seemed up to date also regarding renamed titles was

/var/mobile/Applications/iBooks/Documents/BKLibrary_database/iBooks_v*.sqlite


This file can be opened with iFile's SQLite Editor and under the Key ZBKBOOKINFO there's the mapping between filenames and book/pdf titles.