XLS files are Excel workbook files created in Excel 97 or Excel 2003. The 2007, 2010 and 2013 versions of the program use the XLSX format by default to save workbooks, but can also use the XLS format.

Upgrade to XLSX for smaller file sizes, enhanced security, modern collaboration features, and to prevent accidental data loss.

Microsoft Excel has several features designed to help you recover unsaved versions of spreadsheets and prevent data loss. If Excel closes before you save a file, you should see a Document Recovery tab ...