Ask HN: What do you use for writing and delivering documents in many formats?

I work in a space where more traditional customers often want physical documents - PDFs, or even printed documents - while others ask for things like readthedocs-esque sites. What tools do you use for this, where you can write documentation in one place, and deliver multiple kinds of end-user formats?

Note that this isn't just code docs, but procedural/informative/tutorial type docs as well.