DOCX is supposedly an open standard too, the thing is neither Microsoft nor Adobe fully follow it and instead opt to make it slightly incompatible in order to make it hard for competitors to use it without issues.

EEE tactics.

The PDF standard is so complex that it makes it implicitly not really open, as it's basically impossible to implement it fully

My very cursory understanding is that there is a subset of PDF that is open, something called PDF-A I think, at least enough that it's used for archival. Source: my fingers, fingling on the keyboard.

DjVu probably but it's old and nobody uses it. PDF isn't ideal seems to be workable enough, it was always a much more functional way to share documents than doc or docx.

You want https://en.wikipedia.org/wiki/PDF/A

Pretty sure odf extension is what LibreOffice uses for a "Word" doc, so I would hope it's open. Now whether or not MS properly uses it is another thing entirely.