pet open source projects that no one else ever seems to contribute to, not […] software that holds up civilization

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pandoc.org is probably what you are looking for, but you might have to create a custom reader/writer or find one on the internet.

Shameless plug for Pandoc because I love it

That scalable vector graphic on the page shows source document type on the left and target type on the right. TL;DL: It converts about two dozen document types into about three dozen document types.

P.S.E.G.: PDF ← Markdown ←→ HTML → PDF

P.P.S: Where are my manners? Image transcription added to post.