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.

Have you ever sent an Excel file to a colleague, only to realize they couldn’t open it because of compatibility issues? Or perhaps you’ve spent hours perfecting a spreadsheet, only to see its ...