To open a .docx file, you must use Microsoft Word 2007 or later versions. If you do not have one of these versions and still need to view a .docx file, you can download a free DocX viewer application, or a compatibility pack from Microsoft.
Earlier versions of Microsoft Word defaulted to the .doc file extension.
What is the extra "x" for?
The extra "x" in the Microsoft Office .docx, .xlsx, and .pptx file extensions indicates the files are using the XML format.