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.

When a spreadsheet is open in OnlyOffice, I save it as a .xlsx using the graphical option. OnlyOffice appears to do this. It's expected the OnlyOffice data was successfully exported to an .xlsx file.

This example illustrates how to use the DevExpress Spreadsheet Document API to export different reports to different sheets of a single XLSX file. You need a license for the DevExpress Office File API ...