Microsoft has released free Word & PowerPoint viewers to enable computer users to read documents created by the latest version of Microsoft Office, 2007.
Microsoft Office 2007 uses a new Office Open XML format that is not understood by previous versions of the software. Word documents now carry a .docx extension and PowerPoint presentations have become .pptx files. Computer users who do not own Microsoft Office can now view these kinds of documents using one of the following free downloads:
Alternatively, users of Microsoft Office XP and 2003 can view, create, edit, and save documents in these new formats by installing the Microsoft Office Compatibility Pack.
Source: Digital Inspiration.