pnm2ppa, a PPM to PPA converter, creates output using the PPA (printer
performance architecture) protocol.  This protocol is used by some HP
"Windows-only" printers, including the HP Deskjet 720C series, the HP
DeskJet 820 series, and the HP DeskJet 1000 series.  It has been
tested on all three series, but your personal experience (positive or
negative) is very much appreciated!