This program is part of Netpbm.
Starting with Netpbm 10.86 (March 2019), ppmbrighten is obsolete. Use pambrighten instead. pambrighten is mostly backward compatible with ppmbrighten. You can use the pambrighten manual for ppmbrighten as long as you ignore features that were added after Netpbm 10.86.
One way in which pambrighten is not backward compatible is that pambrighten does not have a -normalize option. If you want that function, use pnmnorm on the image before passing it to pambrighten.
Another backward incompatibility is that pambrighten produces the same kind of output as the input, whereas ppmbrighten always produces PPM. If you need PPM output, pass the results through ppmtoppm.
ppmbrighten is currently implemented as a triviel program that invokes pambrighten, pnmnorm, and ppmtoppm.