In your page 'https://demos.telerik.com/aspnet-ajax/spreadsheet/examples/import-export/defaultvb.aspx',
I can't import a xls file.
I export yours example from the page with name 'Workbook.xlsx'. Then I convert this file to 'Workbook.xls'. In this new file I had removed all formats to avoid problems with compatibility. Finally I import file in your page and I export it. I can open this file in excel.
¿Your spreadsheet runs with xls files?
My goal is to export xls files to PDF, either directly or by converting them previously to xlsx files.
Thanks in advance