Fbreader doesn't support isilos *.pdb books. Remember, .pdb is a generic extension, internally the files can be almost anything. It only supports some *.pdbs.

See the fbreader about page for filetypes supported - it's a great program, just doesn't do isilo. It claims to do non-DRM'ed mobipocket, but I've never been able to get it to work with the few files I have.