Merge pdf with digital signature

Good, I have several pdf with digital signature and when doing the merge does not keep it, I use the following code, which modification I can make to keep it.

Thank you very much

 public static void MergeFiles(string destinationFile, string[] sourceFiles, string[] pageName)
    {
        try
        {
            int f = 0;

            PdfReader reader = new PdfReader(sourceFiles[f]);

            int n = reader.NumberOfPages;

            Document document = new Document(reader.GetPageSizeWithRotation(1));

            PdfWriter writer = PdfWriter.GetInstance(document, new FileStream(destinationFile, FileMode.Create));

            document.Open();
            PdfContentByte cb = writer.DirectContent;
            PdfImportedPage page;

            int rotation;

            int pdfPageName = 0;
            bool pdfNewPageFlag = true;
            while (f < sourceFiles.Length)
            {
                int i = 0;
                while (i < n)
                {
                    i++;
                    document.SetPageSize(reader.GetPageSizeWithRotation(i));
                    document.NewPage();
                    if (pdfNewPageFlag == true)
                    {
                        Chapter chapter = new Chapter(pageName[pdfPageName], pdfPageName + 1);
                        document.Add(chapter);
                        pdfNewPageFlag = false;
                        pdfPageName++;
                    }

                    page = writer.GetImportedPage(reader, i);
                    rotation = reader.GetPageRotation(i);

                    if (rotation == 90 || rotation == 270)
                    {
                        cb.AddTemplate(page, 0, -1f, 1f, 0, 0, reader.GetPageSizeWithRotation(i).Height);
                    }
                    else
                    {
                        cb.AddTemplate(page, 1f, 0, 0, 1f, 0, 0);
                    }
                    Console.WriteLine("Processed page " + i);

                }
                f++;
                if (f < sourceFiles.Length)
                {
                    reader = new PdfReader(sourceFiles[f]);

                    n = reader.NumberOfPages;


                    Console.WriteLine("There are " + n + " pages in the original file.");
                    //
                    pdfNewPageFlag = true;
                }
            }

            document.Close();
        }
        catch (Exception e)
        {
            Console.Error.WriteLine(e.Message);
            Console.Error.WriteLine(e.StackTrace);
            Console.ReadKey();
        }
    }