Also, you can use PDFCombineProX at https://www.coolutils.com/PDFCombineProXNET
string src="C:\\test\\test1.docx".Chr(13)."C:\\test\\test2.xlsx";
string dest="C:\\test\\DestCombine.PDF";
PDFCombineProX Cnv = new PDFCombineProX();
Cnv.Convert(src, dest, "-c PDF -log c:\\Combine.log");
MessageBox.Show("Combine complete!");