Microsoft Open XML Converter for Mac

From Microsoft: Open XML Converter allows you to convert Open XML files that were created in Office 2008 for Mac or Office 2007 for Windows so that you can open, edit, and save them in earlier versions of Office for Mac. Open XML Converter can convert Word documents, Excel workbooks, and PowerPoint presentations that are in the Open XML Format so that you can open and edit the files in Office 2004 for Mac and Office v. X for Mac. You can choose to convert and open one file, or convert a large number of files.