If you are using Microsoft Office 2007 or later, you will be notified when opening the file that the file was not made in Excel, which is normal. Just click “Yes” to continue opening the file.
Note: To use the file in any other spreadsheet application, or online in Google Docs, it is recommended to open the file in Microsoft Excel, then re-save it as a XLS or XLSX file, and use that newly saved file for third parties. If you do not have Microsoft Excel, a free alternative is OpenOffice.