With the move to Microsoft Office 2008 on the Mac (and Microsoft Office 2007 on the PC) a new format has been introduced for saving files in called Open XML with file extentsions .docx, xlsx, and .pptx. This new format brings some advantages to Microsoft Office, however if you are working with users who do not have the most current version of Microsoft Office, they will be unable to work on these files.
By default, all new documents will save in the new formats, however existing documents will always remain in the format they were already in
To save a file using the older Office formats (.doc, .xls, .ppt) follow these steps:
1. When you are ready to save your document, go to File -> Save As
2. Under the Format drop down, choose the option that says 97 - 2004 document. This will look the same for Word, Excel and Powerpoint
3. Click Save. After performing the Save As on a document into the older format, future saves of that document will remain in the old format.