This program is part of Netpbm.

Starting with Netpbm 10.3, pnmarith is obsolete. Use

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.

netpbm documentation 22 June 2002 Pnmarith User Manual(0)