The only working viewer I've found is on the HP Laserjet 3300 installation disk, which didn't require the printer to be connected for the installation to start.
The install disk created the 'c:\Program Files (x86)\Hewlett-Packard\' and within there, hpphpd.exe is the HPD viewer.
Once opened in the viewer, if you have Acrobat or any other a free PDF print driver, you can simply print the files to PDF.