B&N ebooks come in two flavors, ePub with password DRM and eReader (also with password DRM). The former has extension .epub the latter .pdb. If you have eReader, you might try downloading again because many ebooks have recently shifted to ePub. For ePub, see
Barnes & Noble DRM for EPUB circumvented and the resulting DRM-free ePub should work on your PRS-600. If you have eReader ebooks, then ereader2html.py can remove the DRM and then Calibre can convert to ePub.