iTextSharp Error: The Document is not open in C# .Net

i need to add a tittle to my pdf but when i try the pdfcontentbyte 

it's throw a "the Document is not open" error :/

how can i solve it (here a bit of my code where i declare and everything BUT some words are in spanish)

THANKS :)

 Dim sr As New StringReader(sw.ToString())
                Dim pdfDoc As New Document(PageSize.B3, 10.0F, 10.0F, 300.0F, 0.0F)
                Dim htmlparser As New HTMLWorker(pdfDoc)
                PdfWriter.GetInstance(pdfDoc, Response.OutputStream)
                Dim cb As PdfContentByte
                Dim linea As PdfContentByte
                Dim rectangulo As PdfContentByte
                Dim fuente1 As iTextSharp.text.pdf.BaseFont

                pdfDoc.Open()

                cb = pdfw.DirectContent
                linea = pdfw.DirectContent
                rectangulo = pdfw.DirectContent

                'Agregamos una pagina.


                'Iniciamos el flujo de bytes.
                cb.BeginText()




                fuente1 = FontFactory.GetFont(FontFactory.HELVETICA, iTextSharp.text.Font.DEFAULTSIZE, iTextSharp.text.Font.NORMAL).BaseFont 'fuente negrita
                cb.SetFontAndSize(fuente1, 8) 'llamada a la fuente y tamaño
                'Seteamos el color del texto a escribir.
                cb.SetColorFill(iTextSharp.text.BaseColor.BLUE)
                cb.ShowTextAligned(PdfContentByte.ALIGN_CENTER, "TEXTO de prueba (centro)", 315, 730, 0)



                ' Agregamos una pagina.
                pdfDoc.NewPage()