DOCX is a package, not one text stream
A DOCX file is a ZIP-based Open Packaging Convention package. Its parts can include the main document, styles, numbering, headers, footers, comments, images, embedded objects, and relationship files. Applications use those parts together to render and edit the document.
How to open a DOCX file
Microsoft Word is the format's best-known editor. Other office applications and browser-based editors can open many DOCX files, although complex layout, fonts, fields, macros, or embedded objects may behave differently. Keep an original copy before using a conversion or repair service.
Structure matters for accessibility
DOCX can store heading styles, list semantics, table headers, document language, image alternatives, link targets, and other useful information. Whether those features are present and meaningful depends on authoring. A paragraph made large and bold is not automatically a heading.
The filename extension does not prove quality
Two files with the same DOCX extension can differ greatly in structure and accessibility. Test the document in its authoring application, inspect its reading and navigation behavior, and test any PDF created from it. Export is a separate transformation that can introduce new defects.