Releases after 10.34 are not available via the Sourceforge file release
facility.

You can get later releases via the Sourceforge Subversion facility,
as described on the Netpbm web site: <http://netpbm.sourceforge.net/> .

It boils down to this shell command for most cases:

  $ svn checkout https://netpbm.svn.sourceforge.net/svnroot/netpbm/advanced netpbm


Alternatively, current Super Stable releases are distributed via
tarball as a Sourceforge file release.  These are typically 1-3 years
behind in features.