The Signum! word processor was a text editing application from the german software publisher "Application Systems Heidelberg" (ASH), written for the ATARI ST. It was one of the most popular word processors available for that system. This document is as far as I know the only description of that file format that is available online.
Every SDO file starts with the bytes 73 64 6f 63
, that is sdoc
in most ASCII-compatible