HP DeskJet 720C is bad printer for Linux
PPA (Printing Performance Architecture) is a closed, proprietary protocol developed by Hewlett Packard for a short-lived series of DeskJet printers. In
essence, the PPA protocol moves the low-level processing of the data to the host computer rather than the printer. This allows for a low-cost (to
produce) printer with a small amount of memory and computing power. However, in practice the printer was often as expensive as more capable
printers and HP has since discontinued the use of PPA in favour of returning to PCL3e in their latest USB-based printers.
If you want to use your printer under Linux, you are much better off getting a PCL or Postscript printer.
Have a look at: