File type PBM is graphical format carrying name Portable Bitmap Format. It has got many derivations, for example Portable Graymap Format (PGM) or Portable Pixmap Format (PPM). Its advantage, today it is probably disadvantage, it can use only 2 colors that are black and white.
It was announced in 80s and its creator is Jef Poskanzer. Originally it was used for sending text pictures, which could handle text formatting. This was used especially in e-mail communication. In 1988 Poskanzer created package of tools for this file format, this package allowed even to transfer this format into another. At the end of the same year was released format PGM and PPM together with their tool packages. Both of them were packed into library called Pbmplus. In 1993 was Pbmplus replaced by Netpbm, which was repair and repack of these packages into one.
To open these files you can use Adobe Photshop that is not unfortunately for free, but there is trial version which is 30-day long.