pbmtext

The documentation can be viewed here

http://netpbm.sourceforge.net/doc/pbmtext.html

For more complex operations, the page above says

"*pbmtext* is meant for small quantities of simple text. If you're working with a /document/, you would be better off using a document formatting program to "print" to a Postscript file, then feeding that Postscript to *pstopnm*."

A simple example is here:

pbmtext test.pbm

pnmtojpeg test.pbm >test.jpg

or in one line

pbmtext test.jpg