Historically, a2ps started as a text to PostScript converter, but thanks to powerful delegations it is able to let you use it for any kind of files, ie it can also digest manual pages, dvi files, texinfo, ....
? h The control sequence at the end of the top line of your error message was never \def'ed. If you have misspelled it (e.g., `\hobx'), type `I' and the correct spelling (e.g., `I\hbox'). Otherwise just continue, and I'll forget about whatever was undefined.