A .docx is a zip archive of XML. Word opens it, and so do several things that are free.
A `.docx` is not a single document but a compressed folder: rename it to `.zip` and you can open it to find the text, the styles and the images as separate files inside. That is the format, and it is why a `.docx` cannot be read usefully in a text editor.
Google Docs opens a `.docx` by uploading it to Drive, LibreOffice Writer opens it on any desktop system and is free, and both Apple Pages and Microsoft’s own web version of Word open one without a paid licence.
On a phone, the Word app opens `.docx` files for reading without a subscription; editing is where the paywall starts.
A file that arrives as `document.docx` but refuses to open in anything is often a `.doc` — the older format — with the wrong extension, or a file that did not finish downloading. Check the size first: a truncated download is usually obviously too small.
A password-protected `.docx` will open the dialogue and nothing else. No converter can pass that, which is a property of the file rather than a limitation of the tool.
Converting to Markdown keeps the headings, lists, links and tables and throws away fonts, margins and page breaks. For text that has to live in a repository, a wiki or a diff, that trade is the point rather than a loss.