public static byte[] GetPDF(IEnumerable<string> lista)
        {
            int numberOfPages, currentPageNumber, rotation;

            MemoryStream ms = new MemoryStream();
            Document doc = new Document();



            PdfWriter writer = PdfWriter.GetInstance(doc, ms);
            doc.Open();
            PdfContentByte cb = writer.DirectContent;
            PdfImportedPage page;


            foreach (string f in lista)
            {
                //--- safety code
                if (!File.Exists(f))
                    return null;


                byte[] bytes = File.ReadAllBytes(f);


                PdfReader reader = new PdfReader(bytes);


                numberOfPages = reader.NumberOfPages;
                currentPageNumber = 0;

                while (currentPageNumber < numberOfPages)
                {
                    currentPageNumber += 1;
                    doc.SetPageSize(reader.GetPageSize(currentPageNumber));
                    doc.NewPage();
                    page = writer.GetImportedPage(reader, currentPageNumber);
                    rotation = reader.GetPageRotation(currentPageNumber);

                    if (rotation == 90 || rotation == 270)
                        cb.AddTemplate(page, 0, -1.0F, 1.0F, 0, 0, reader.GetPageSizeWithRotation(currentPageNumber).Height);
                    else
                        cb.AddTemplate(page, 1.0F, 0, 0, 1.0F, 0, 0);

                }
            }

            doc.Close();

            return ms.ToArray();
        }