This program is part of Netpbm.
pnmcomp was obsoleted by pamcomp, introduced with Netpbm 10.21 (March 2004). pamcomp is backward compatible with pnmcomp, plus adds many additional functions. Among this is the ability to process PAM images, including those with transparency channels.
pnmcomp remained in the Netpbm package until Netpbm 10.68 (September 2014) because of hopes that it had fewer bugs than pamcomp because of its age. But now it would just be clutter.
In Netpbm before 10.21, use the manual for pamcomp with pnmcomp. Features that are in pamcomp but not pnmcomp are indicated by statements that they didn't exist before 10.21.