Home → Microsoft Office → Office 2003 → Open an Office 2010 and 2007 documents in an earlier version of Office
The 200/2010 Microsoft Office system introduces a new file format that is based on XML. It is called Open XML Formats and applies to Microsoft Office Word 2007/2010, Microsoft Office Excel 2007/2010, and Microsoft Office PowerPoint 2007/2010. This article summarizes key benefits of the new format, describes what the new file name extensions are, and discusses how you can share the 2007/2010 Office release files with people who are using earlier versions of Office.
If you send one of these files to someone with an earlier version of, let's say, Word, then they will need the following on installed on their computer from Microsoft. It's a free utility that allows opening these new document formats on an older verion.
http://www.microsoft.com/en-us/download/details.aspx?id=3
The above link is the page on Microsoft's site with more information and the utlity.