a) If you already have CAT troff and you don't need pic or grap:

       Your simplest bet is psroff if you want to drive printers supported
       by psroff (PostScript, HP Laserjet, ditroff filters, PBM+
       supported devices (including Epson 24 pin, FAX).  Transcript
       is an alternative for PostScript output (but doesn't
       allow you to mount fonts, no drawing support).

    b) You already have ditroff:

       You may already have a driver for your printer.  Check.
       If not, your best bets are jetroff, psroff, tpscript,
       transcript.
    
    c) You don't have a troff or, you have CAT troff and you need pic
       and grap:

       Get "a DWB", such as groff (no grap), DWB 3, or one of the
       commercial variants.