Good to know
DOCX vs DOC: why only one opens
.docx is the format Word has used since 2007: a zipped bundle of XML files that describes text, styles and images. Because the format is documented, a browser can read it and lay it out without Word.
The older .doc format is a closed binary format from the 1990s. Browsers cannot reliably read it, so this viewer does not try. If you have a .doc file, open it in any word processor — LibreOffice, Pages, Google Docs — and save it as .docx.
Complex features such as tracked changes, embedded spreadsheets or exotic fonts may look slightly different from Word. For reading and checking a document, the result is usually indistinguishable.