.docx is the Office Open XML file format that was introduced in Microsoft Office 2007.  Microsoft uses this XML-based format to store all text, formulas, graphics, etc  for Word, Excel, and PowerPoint. The ultimate goal for this switchover is to facilitate extensibility and interoperability by allowing third-party vendors to read these files. So how to open a .docx file?

• If you have Office XP or Office 2003, you must download the Compatibility Pack from Microsoft. Click here to download.
• If you do not have any Microsoft Office product, first download the free Word Viewer application and then the Compatibility Pack from the bullet item above.

And there you have it, a 100% free and legit way to open .docx files!