Microsoft's Office 2007 introduced a new file format for each application within the Office suite. Using the new formats can pose a problem for users who do not have Office 2007, as the new files are not backwards-compatible. This means older versions of MS Office cannot natively open the new file formats.
To ensure all users will be able to open your documents, we recommend saving as a 97-2003 compatible format. Please read for more information on saving backwards-compatible files in Word. The same steps can be applied to the other applications in the Office suite.