> знаю как .doc, а .docx вполне себе обратно совместим.

Угу, почти:
https://joinup.ec.europa.eu/community/osor/case/complex-sing...
> One of the major feature improvements of Writer 4.3 will be the handling of shapes in files imported from MS Office
> documents using the docx import filter. This is no easy task, however, because Vajna has shown that Office itself interprets
> the same file differently: where Office 2010 shows a green triangle, Office 2007 has a red one. And this is just one example.

...
> The problem of opening files from only four years ago
> It is clear that there are several problems related to interpreting files created just four years ago, Lundell says.
> This is, of course, even more complex given the issues and confusion related to the Strict versus Transitional versions of the
> OOXML standard. If a user created a file in MS Office 2007 and saved it in Transitional docx format, other people should be
> able to open it in Office 2013, re-save it in Strict docx and finally read it in Office 2010, he says. But you’d be surprised at the
> real outcome.

А еще там три разных версии: ecma, transitional и strict и между собой они не очень-то совместмы. Ну и сами спеки где-то на 6-7 тысяч страниц (тот же ODF почему-то  раз в 8-9 полегче будет).