Hey gurgle!!! Fancy seeing you out here in the Nook owner's world! Hope you have the chance to visit more, it's a beautiful tab.The reason it likely will not read the pdb is the pdb is either DRM or a non-standard format. Not all PDB are created the same. Try one the can read PRC files or MobiReader.
Also version can make a difference I found. I am still on ireader 1.07 as it is more flexible than 1.08+