pbmreduce − see http://netpbm.sourceforge.net/doc/pbmreduce.html
pbmreduce is part of the Netpbm package. Netpbm documentation is kept in HTML format.
Please refer to <http://netpbm.sourceforge.net/doc//pbmreduce.html>.
If that doesn’t work, also try <http://netpbm.sourceforge.net> and emailing Bryan Henderson, [email protected].
Note that it is possible to install Netpbm with the documentation available differently. For example, you could simply see the documentation instead of the message you are reading now. The file ’doc/USERDOC’ in the Netpbm source tree contains details.