Add image to PDF with iTextSharp and VB.Net

Hi Experts

I have the following code using iTextSharp.   It creates a barcode and inserts it into a PDF document created by iTextSharp

The code works correctly but only with newly created documents.   I need to be able to do the same thing but with an already existing PDF.   I have searched and found many examples but all assume a newly created document which doesn't help me.

Any assistance would be appreciated

        Dim Batch As String = "12345"

        'Define a new PDF Doc
        Dim doc As New Document(New iTextSharp.text.Rectangle(350, 400), 5, 5, 1, 1)  ''the size of the rectangle width x height

        Try
            Dim writer As PdfWriter = PdfWriter.GetInstance(doc, New FileStream("C:\somepath\TestLabel.abc", FileMode.Create))
            doc.Open()

            Dim dt As New DataTable()
            dt.Columns.Add("Batch")
            Dim row As DataRow = dt.NewRow()
            row("Batch") = Batch.ToString    '& i.ToString()
            dt.Rows.Add(row)
          
            Dim img1 As System.Drawing.Image = Nothing
            For i As Integer = 0 To dt.Rows.Count - 1
                If i <> 0 Then
                    doc.NewPage()
                End If

                Dim cb As iTextSharp.text.pdf.PdfContentByte = writer.DirectContent
                Dim bc As iTextSharp.text.pdf.Barcode128 = New Barcode128() 
                bc.TextAlignment = Element.ALIGN_LEFT
                bc.Code = dt.Rows(i)("Batch").ToString()
                bc.StartStopText = False
                bc.CodeType = iTextSharp.text.pdf.Barcode128.EAN13
                bc.Extended = True

                Dim img As iTextSharp.text.Image = bc.CreateImageWithBarcode(cb, iTextSharp.text.Color.BLACK, iTextSharp.text.Color.BLACK)

                cb.SetTextMatrix(100.5F, 320.0F)
                img.ScaleToFit(240, 600)
                img.SetAbsolutePosition(5.5F, 320) 
                cb.AddImage(img)
            Next i

            doc.Close()

            Catch ex As Exception
                doc.Close()
                MsgBox(ex.ToString)
            End Try

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