DOCX
Word documents unzipped: XML all the way down.
Where it came from
DOCX replaced the binary .doc in Office 2007 as part of Office Open XML (ECMA-376). The move was partly forced: governments and competitors demanded open formats, and a zip of XML was Microsoft's answer — proprietary in spirit but at least parseable by anyone.
It became the default business-document format worldwide, for better and worse.
How it works
A DOCX is a ZIP whose heart is word/document.xml: paragraphs (〈w:p〉) contain runs (〈w:r〉), runs contain text (〈w:t〉) with formatting properties. Styles, numbering, headers and images live in sibling parts linked by relationships.
Because the content model maps cleanly to HTML (paragraph → 〈p〉, run → 〈span〉, bold → 〈strong〉), DOCX→HTML conversion is nearly mechanical — that's what UWC does via mammoth, plus a plain-text mode that strips everything but words.
Convert DOCX
DOCX to clean HTML or plain text — parsed server-side, your documents aren't stored.