Work through this in order
- Check whether the file is a valid ZIP container
- Find [Content_Types].xml and word/document.xml
- Review media and relationships in the package listing
- Use Office or LibreOffice for accurate document layout
- Avoid enabling macros or external links from unknown files
Use the matching tool
Inspect a local copy with DOCX File Inspector. After the check, save or repair files only using trusted software that supports the format.