This program is part of Netpbm.
Starting with Netpbm 10.3, pnmarith is obsolete. Use pamarith instead. pamarith is backward compatible with pnmarith except where your input images have different depths. pnmarith would convert the one with the lesser depth to have the higher depth before doing the arithmetic. With pamarith, you must do that step separately, using pgmtoppm.