Pandoc

Pandoc is a free and open-source document converter, widely used as a writing tool (especially by scholars)[2] and as a basis for publishing workflows.[3] It was created by John MacFarlane, a philosophy professor at the University of California, Berkeley. https://en.wikipedia.org/wiki/Pandoc

The input format with the most support is an extended version of Markdown.[9] Notwithstanding, Pandoc can also read in the following formats: (long list: epub, docbook, markdown/commonmark, OPML, etc.)

Output format: roughly the same as the inputs: also AsciiDoc, PDF (using 3rd party plugin)

Written in Haskell.