In a project I worked on many moons ago we were writing documentation in the APT format, and publishing to HTML and PDF using a Maven-based toolchain.
APT served us well, but it hasn’t been supported or improved by the community in a long time. When the time came to update the documentation for a major release, we decided to switch to using Markdown, which was a format everyone was already familiar with, and allowed the team to take advantage of all the tools, such as Sublime plugins, that support Markdown.
Converting APT documents to Markdown is a two step process of APT -> XHTML -> Markdown using the Doxia converter which can be downloaded here and the excellent swiss-army document format conversion tool Pandoc:
# Converting over the existing APT docs to XHTML via Doxia converter
> java -jar ~/Downloads/doxia-converter-1.2-jar-with-dependencies.jar -in your_doc.apt \
-from apt -out ./ -to xhtml
# Convert resulting XHTML to Markdown
> pandoc -o your_doc.md your_doc.apt.xhtml
The end result will require a bit of manual fixing up, but in my experience it was pretty minimal and beats doing it manually or writing your own converter.