using iTextSharp.text;
    using iTextSharp.text.pdf;

    public class PageNumberHelper : PdfPageEventHelper
    {
        private PdfContentByte cb;
        private BaseFont bf;

        public override void OnOpenDocument(PdfWriter writer, Document document)
        {
            try
            {
                bf = BaseFont.CreateFont(BaseFont.TIMES_ROMAN, BaseFont.CP1252, BaseFont.NOT_EMBEDDED);
                cb = writer.DirectContent;
            }
            catch (DocumentException deex)
            {
                //...
            }
            catch (IOException ioex)
            { 
                //...              
            }
        }

        public override void OnEndPage(PdfWriter writer, Document document)
        {
            base.OnEndPage(writer, document);
            String text = writer.PageNumber.ToString();
            Rectangle pageSize = document.PageSize;
            cb.BeginText();
            cb.SetRGBColorFill(100, 100, 100);
            cb.SetFontAndSize(bf, 9);
            cb.SetTextMatrix(pageSize.GetRight(30f), pageSize.GetBottom(20f));
            cb.ShowText(text);
            cb.EndText();
        }
   }

       //..................................
        var document = new Document(PageSize.A4, 17f, 17f, 28.33f, 28.33f);
        using (var pdfStream = new MemoryStream())
        {
             var writer = PdfWriter.GetInstance(document, pdfStream);
             writer.ViewerPreferences = PdfWriter.PageModeUseOutlines;
             writer.PageEvent = new PageNumberHelper();
             //..................................
             document.Close();
             return pdfStream;
         }