iBooks importing - does it duplicate files?

Just getting into iBooks and enjoying the app.

Question, when iBooks imports books from either the apple store or my own pdfs, does it create a duplicate file and store it?

Or does the program app 'locate' the files on the hard drive and simply read them?

I am asking this question because iTunes gives a person an option to import without duplicating the file. I cannot find a similar preference in Ibooks. Just trying to save on hard drive space. I understand that obviously any downloads from App store would be stored within a contents folder in Ibooks.