Proportionally reduce a PBM image. See also:
pamenlarge,pamditherbw. More information: https://netpbm.sourceforge.net/doc/pbmreduce.html.
pbmreduce {{n}} {{path/to/image.pbm}} > {{path/to/output.pbm}}
pbmreduce {{[-t|-threshold]}} {{n}} {{path/to/image.pbm}} > {{path/to/output.pbm}}
pbmreduce {{[-va|-value]}} {{0.6}} {{n}} {{path/to/image.pbm}} > {{path/to/output.pbm}}