Arto Viitanen (av@ohdake.cs.uta.fi) has used djgpp (the MSDOS port of
gcc 2.01) to compile groff-1.05 for MSDOS. The binaries are available
at garbo.uwasa.fi in /pc/unix. The file is groff396.zip and it's 725 Kb.
A port of version 1.07 by Mauro Condarelli is available from omnigate.
clarkson.edu in pub/msdos/djgpp/pub.  This port includes the groff
driver and also the dvi device, both of which were missing in the 1.05
port.  Both ports work well except for gpic which does not seem to work on
systems lacking a math coprocessor.

There's also a OS/2 port using the EMX environment (GCC 2.3.3), which
includes a DOS extender.  The latest version may support DOS directly.

Elan, Image Network, and SoftQuad have commercial versions of DWB
for DOS machines.  They tend to be a bit pricey - in the $500-$1200
range.

There are many freeware versions of nroff that have either already
been ported, or should be easy to port to DOS.  For example, C
implementations of Kernighan and Plauger's "Software Tools" "roff"
(see Ozan Yigit's implementation available from comp.sources.unix).
[roff isn't a complete implementation of nroff, depends on what
you need.]

Related: Ghostscript can be used on DOS to preview the output of any
PostScript-producing application, including groff.  If you do use
GhostScript, make sure that you get the latest version with the good
fonts.  GhostScript also contains CGA and VGA drivers for on-screen
preview.

Cawf2 a C port of Henry Spencer's Amazing Workable Formatter (originally
an awk program).  It apparently does a reasonable job with -ms and -man
macros.  The C version is by Vic Abell and is available from ftp.cc.
purdue.edu in pucc/cawf.  It works on MSDOS and most or all UNIX systems.

I'd appreciate any other leads on DOS nroff/troffs.