Ecrit dans un PDF

Bonjour,
je modifie un PDF en utilisant la DLL iTextSharp.
le problème est que lors que Ecrit dans le fichier le texte que je rajoute se retrouve derrière l'image de fond du fichier.
est-ce que vous savez comment je peux le mettre au premier plan ?

Code:

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  Dim newFile As String 
        Dim y As Single = System.Configuration.ConfigurationSettings.AppSettings("y")
        Dim x As Single = System.Configuration.ConfigurationSettings.AppSettings("x")
 
        Dim reader As PdfReader = New PdfReader(pdf_debut)
        Dim size As Rectangle = reader.GetPageSizeWithRotation(1)
        Dim document As Document = New Document(Size)
        Dim fs As FileStream = New FileStream(newFile, FileMode.Create, FileAccess.Write)
 
        Dim writer As PdfWriter = PdfWriter.GetInstance(Document, fs)
        Document.Open()
 
        Dim cb As PdfContentByte = writer.DirectContent()
 
        Dim bf As BaseFont = BaseFont.CreateFont(BaseFont.HELVETICA, BaseFont.CP1252, BaseFont.NOT_EMBEDDED)
 
        cb.SetColorFill(BaseColor.BLACK)
        cb.SetFontAndSize(bf, 8)
        ' write the text in the pdf content
 
 
        cb.BeginText()
 
        Dim text As String = "Some random blablablabla..."
        ' put the alignment and coordinates here
 
        cb.ShowTextAligned(PdfContentByte.ALIGN_CENTER, text, x, y, 0)
 
        cb.EndText()
 
        Dim page As PdfImportedPage = writer.GetImportedPage(reader, 1)
 
        cb.AddTemplate(page, 0, 0)
        ' close the streams and voil� the file should be changed :)
        Document.Close()
        fs.Close()
        writer.Close()