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Files saved in Office 2007 are in a new format by default. They are unreadable by individuals not using Office 2007 as well. To change this behavior for Word go to:
Office button > Word Options > Save > Save files in this format: Word 97-2003 Document (*.doc)
Follow the same procedure in other Office applications to change the way they save files as well.
If you are an Office 2003 user, there is a compatibility pack you can download and install that will enable you to open native Office 2007 files.
The Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats is available here.
Unfortunately, there is no current way for Office 2004 users to open native Office 2007 files. Users of Office 2007 will have to save the files in a previous version format (outlined at the top of the page) for Macintosh users to view and modify the Office files.
Office 2008 for the Mac is scheduled to be delivered in the second half of 2007. This latest version of the Office suite for the Mac will use the Office Open XML formats natively, making it compatible with Office 2007. For more information please see http://www.microsoft.com/presspass/press/2007/jan07/01-09MacworldPR.mspx?source=rss&WT.dl=0.