DOCX is Microsoft Word's format, but you absolutely don't need Word to work with it. The format is based on an open standard (Office Open XML), and several free tools handle it well.
Google Docs (free, browser-based) opens DOCX files with good formatting fidelity. Upload to Google Drive, double-click to open, and you're editing in seconds. Track changes, comments, and most formatting survive the conversion. This is the best option when you need to edit and collaborate — Google Docs' real-time collaboration is arguably better than Word's.
LibreOffice Writer (free, cross-platform) is the best desktop alternative. It opens DOCX files natively and preserves most formatting, including headers, footers, tables, and embedded images. Complex documents with advanced features (mail merge, embedded macros, some table styles) may render slightly differently than in Word, but for 95% of documents it's indistinguishable.
Apple Pages (free on Mac/iPad/iPhone) opens and exports DOCX. The formatting translation is generally good, though complex layouts may shift. For iPhone and iPad users, it's the most convenient option.
For quick viewing without editing, most web browsers can preview DOCX files. Chrome, Edge, and Firefox will display the content when you drag a DOCX file into the browser window, though formatting may be simplified. macOS Quick Look (press Space in Finder) also renders DOCX files for preview.