This program is part of Netpbm.
pnmscale was obsoleted by pamscale, introduced with Netpbm 10.20 (January 2004). pamscale is backward compatible with pnmscale, plus adds many additional functions, including the ability to process PAM images.
pnmscale remained in the Netpbm package until Netpbm 10.46 (March 2009) because of hopes that it had fewer bugs than pamscale due to its age. But now it would just be clutter.
In Netpbm before 10.20, use the manual for pamscale with pnmscale. Features that are in pamscale but not pnmscale are indicated by statements that they didn't exist before 10.20.