.TelerikNormal {font-family: Calibri;font-size: 14.6666666666667px;margin-top: 0px;margin-bottom: 0px;line-height: 100%;color: #000000;}

.TelerikHeading3 {font-family: Calibri Light;font-size: 22.6666666666667px;}

provider.ExportSettings.StylesExportMode = StylesExportMode.Inline;

<p style="font-family: Calibri;font-size: 14.6666666666667px;margin-top: 0px;margin-bottom: 0px;line-height: 100%;color: #000000;"><span>Test</span></p>

            string inputFileName = "input.xlsx";
            if (!File.Exists(inputFileName))
            {
                throw new FileNotFoundException(String.Format("File {0} was not found!", inputFileName));
            }

            Telerik.Windows.Documents.Spreadsheet.Model.Workbook workbook;
            IWorkbookFormatProvider formatProvider = new Telerik.Windows.Documents.Spreadsheet.FormatProviders.OpenXml.Xlsx.XlsxFormatProvider();

            using (Stream input = new FileStream(inputFileName, FileMode.Open))
            { 
                workbook = formatProvider.Import(input, TimeSpan.MaxValue);
            }
            string outputFilePath = "output.xlsx";

            using (Stream output = new FileStream(outputFilePath, FileMode.Create))
            {
                formatProvider.Export(workbook, output, TimeSpan.MaxValue);
            }
            Process.Start(new ProcessStartInfo() { FileName = outputFilePath, UseShellExecute = true });