

docx is OOXML, Open Office XML.
However, it was definitely written to ensure existing doc files could be fully supported, so a lot of the format is “bit 7: if enabled, use Word for Mac 5.0 layout engine.” So… Documented, yes. Usable, no.
PDF, as mentioned, is very similar. The format is available, but no one really wants to implement ALL of it.

Judging my number of neurospicy friends, I might be somewhere on a spiciness spectrum – but I don’t know anyone who doesn’t think trains are awesome!
I was surprised not to see this one, but it’s about laying train tracks, so maybe it doesn’t qualify: Driving the Last Spike by Genesis