DOCX files are the default format for Microsoft Word documents since 2007, using XML-based structures. You may be unable to open one primarily due to software incompatibility (using software too old to understand DOCX), file corruption (transmission or storage errors damaging the file), or insufficient permissions (restricted access via password or system security settings). It differs from the older .doc format which is less efficient and requires specific compatibility packs for older software.

Common scenarios include opening a file created in the latest Microsoft Word version (like Microsoft 365) with a much older version (like Word 2003) that doesn't support the modern XML standards. Alternatively, attempting to open it in basic text editors like Notepad or incompatible office suites like older versions of LibreOffice may also fail to render it correctly.

While DOCX offers benefits like smaller file sizes and improved data recovery over DOC, its complexity makes it prone to corruption. Proprietary aspects can create hurdles for users without specific Microsoft software. Solutions often involve using the free Word Online viewer in OneDrive, ensuring your software is updated to the latest compatible version, repairing the file via Word's "Open and Repair" feature, or requesting an uncorrupted copy from the sender.