This program is part of Netpbm.

Starting with Netpbm 10.32 (Februrary 2006), pnmdepth is obsolete.
Use pamdepthinstead.

pamdepth is backward compatible with pnmdepth. You can use the
pamdepth manual for pnmdepth as long as you ignore features that were
added after Netpbm 10.31.

For backward compatibility, the name 'pnmdepth' continues to exist as
an alias for 'pamdepth'. But due to a bug, that name doesn't work in
Netpbm 10.32. You have to fix the symbolic link.

netpbm documentation 06 March 2006 Pnmdepth User Manual(0)