https://stackoverflow.com/questions/40037905/what-is-the-structure-of-a-docx-and-doc-file
Docx is basically a zip archive with a lot of xml files in it. It is an open format and the documentation is available online. The wikipedia article has a general description and the links you will need.
https://en.wikipedia.org/wiki/Office_Open_XML
实际上将.docx改为.zip打开就能看到里面的内容了，其实非常简单。