Where does iBooks store pdf files

I know when an eBook is downloaded for the iBook store it is stored locally on the Mac as an epub file in the user's library/container/ folder. But I can't find where it stores PDF files that I import from my Mac into iBooks.

I assume it is making a copy of the pdf and putting it somewhere in the library folder. Am I wrong about that? Is it just storing a reference to the original file or is it storing a copy somewhere?

Does anyone know?

Thanks,

Mark