Excel 2007 for Windows and Excel 2008 for Mac will automatically save with a .xlsx extension which cannot be read by previous versions of Office. To avoid this problem, save down — that is, save as an earlier version so that people using older versions of Office can open the file.
To save down in Excel 2007 for Windows
Click the Microsoft Office button, select Save As, and select Excel 97-2003 Document.
To save down in Excel 2008 for Mac
Select File > Save As.
In the Format drop-down menu, choose Excel 97-2004 Workbook (.xls)
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