pamtohdiff is part of the Netpbm package. Netpbm documentation is kept in HTML format.
Please refer to <http://netpbm.sourceforge.net/doc//pamtohdiff.html>.
Note that making the documentation available this way was a choice of the person who installed Netpbm on this system. It is also possible to install Netpbm such that you would simply see the documentation instead of the message you are reading now.