Files with the .docx extension can be opened with Microsoft Word 2007 or later on Windows machines and Word 2008 or later Mac OSX systems. The open-source application Open Office can also open .docx files. Freeware text viewers, such as TextMaker by SoftMaker, also open .docx files.
Some free online file conversion sites can convert .docx files into other formats, such as .doc files. The .docx extension is the Microsoft Office Open XML format, and it replaced the previous .doc used by older versions of Microsoft Word. Versions of Microsoft Word prior to 2007 cannot open files with a .docx extension.