pbmtopsg3 [--title=title]
pbmtopsg3 [--title=title] [--dpi=dpi] [filespec]
This program is part of netpbm(1).
pbmtopsg3 converts the PBM images in the input PBM file to pages in a Postscript file encoded with G3 fax compression.
If you don't specify filespec, the input is from Standard Input.
Remember that you can create a multi-image PBM file simply by concatenating single-image PBM files, so if each page is in a different file, you might do:
cat faxpage* | pbmtopsg3 >fax.ps