private void button1_Click(object sender, EventArgs e)
        {
 
       string[] pdfList = new string[3];
        pdfList[0] = @"C:\Users\smile\Documents\RPT01.pdf";
        pdfList[1] = @"C:\Users\smile\Documents\RPT02.pdf";
 
        mergePDFFiles(pdfList, "1.pdf");
      
 }
          void mergePDFFiles(string[] pdfList,string outMergeFile)
        {
            string mergePDFFiles = @"C:\Users\smile\Documents\1.pdf";
           
            Document DOC=new Document();
            PdfWriter writer = PdfWriter.getInstance(DOC, new FileStream(mergePDFFiles, FileMode.Create));
        
            DOC.Open();
            PdfContentByte cb = writer.DirectContent;
            PdfImportedPage newPage;
 
            for (int i = 0; i < pdfList.Length; i++)
            {
 
                PdfReader reader = new PdfReader(pdfList[i]);
                int iPageNum = reader.NumberOfPages;
                for (int j = 1; j <= iPageNum; j++)
                {
                    DOC.newPage();
                    newPage = writer.getImportedPage(reader, j);
                    cb.addTemplate(newPage, 0, 0);
                }
            }
            DOC.Close();
        }
    }
}