Microsoft Word 2007 and later versions use the DOCX file format. It is designed to replace the DOC file format used in previous versions of Word and utilizes the .docx file extension.
Microsoft Word 2007 or later versions are required to open .docx files. If you don’t have one of these versions but still need to view a .docx file, you can get a free DocX viewer program or a Microsoft compatibility pack.
The additional “x” in the Microsoft Office .docx,.xlsx, and .pptx file extensions indicate that the files are in XML format.