help with converting pbm to png

I have a .pbm file that was extracted from a .pdf file from a
microfilm reader / scanner.  When I run "convert file.pbm file.jpg"
it works fine.  I get back a good image that looks just like it
does in the .pdf file.  However when I run "convert file.pbm
file.png" it is all grainy and almost impossible to read.  I have
tried all kinds of density and other options to no avail.  Any
suggestions on how to get a quality png image would be appreciated.