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 });

16 0 obj
<</AP<</N<</Off null/On 188 0 R>>/D<</Off 189 0 R/On 190 0 R>>>>/AS/Off/F 4/FT/Btn/H/T/P 19 0 R/Rect[ 40.3 690.45 56.15 706.7999]/Subtype/Widget/T(Einraeumung:Grabnutzungsrechts)/Type/Annot>>
endobj

using (PdfStreamWriter fileWriter = new PdfStreamWriter(File.OpenWrite(document2Name)))
{
	// Iterate through the files you would like to merge 
	for (int i = 0; i < 2; i++)
	{
		// Open each of the files 
		using (PdfFileSource fileToMerge = new PdfFileSource(File.OpenRead(document1Name)))
		{
			// Iterate through the pages of the current document 
			foreach (PdfPageSource pageToMerge in fileToMerge.Pages)
			{
				// Append the current page to the fileWriter, which holds the stream of the result file 
				fileWriter.WritePage(pageToMerge);
			}
		}
	}
}

document1.Merge(document2);