Office 2010 and Office 2007 create files in an “OpenXML” format which is not compatible with earlier versions of Office. This page explains how to work with Office 2007 / 2010 files so that people using different versions of Office will be able to read them.
Can an older version of Office read a file created in Office 2007 or Office 2010?
Not necessarily. It is possible for people with Office 2007 and Office 2010 to use the Save As feature to save documents in the older “97-2003″ format. If you use Office 2007 or 2010 and you work with people who may have an older version of Office, consider saving documents you share in this format to ensure that they will be readable.
Compatibility Pack for Windows: If you use a pre-2007 version of Office on a Windows-based PC, you may want to download and install Microsoft’s free Office Compatibility Pack for Windows which allows earlier Windows-based versions of Office to read files created in the new formats.
The Compatibility Pack must be installed by someone with Administrator privileges. If your PC is administered by someone else, you should consult with your system administrator regarding the best solution.
What about Mac users?
Office 2008 for the Mac can read the new file formats, but earlier versions cannot.
If you use a Mac, you will be able to read files created in Windows Office 2007 format most easily if you are using Office 2008, which came out in January 2008.
Earlier versions of Office for the Mac cannot read the new file formats. Microsoft has a free converter for Mac (for Word and PowerPoint only, not Excel) in December 2007. This converter is less than perfect. You will get better results either using Office 2008 or asking associates who use Office 2007 to save their files in the “97-2003″ format.
Apple offers another solution, saying that iWork is compatible with the Office 2007/2010 formats (Word, PowerPoint, and Excel). You can download a free 30-day trial.