iTextSharp: combinar dos PDF y desactivar formularios

[Bravenap]

Buenas. Estoy utilizando iTextSharp (vers. 5.0.2.0) para combinar dos PDF con formularios. Después de rellenar los campos de cada formulario pretendo desactivarlos mediante FormFlattening. Cuando lo hago de manera individual funciona pero si los combino no. Os dejo el código:

Código vb:

Dim ms As New System.IO.MemoryStream()
Dim mod01 As New PdfReader(Server.MapPath("~/Mod01.pdf"))
Dim stp01 As New PdfStamper(mod01, ms)
With stp01.AcroFields
     .SetField("txtCodSolicitud", String.Format("{0:yyyyMMdd}{1}", Now, Session.SessionID.ToUpper))
     .......
End With
        stp01.FormFlattening = True
 
Dim mod02 As New PdfReader(Server.MapPath("~/Mod02.pdf"))
Dim stp02 As New PdfStamper(mod02, ms)
With stp02.AcroFields
     .SetField("txtNum", Me.ddlNum.SelectedValue)
     ........
End With
stp02.FormFlattening = True
 
Dim doc As New Document()
Dim copia As New PdfCopy(doc, ms)
doc.SetPageSize(PageSize.A4)
doc.Open()
copia.Open()
Dim page As PdfImportedPage
Dim n As Integer
For n = 1 To mod01.NumberOfPages
     page = copia.GetImportedPage(mod01, n)
     copia.AddPage(page)
Next
 
For n = 1 To mod02.NumberOfPages
     page = copia.GetImportedPage(mod02, n)
     copia.AddPage(page)
Next
 
copia.Close()
doc.Close()
 
'Se envía al cliente
Response.ClearContent()
Response.ClearHeaders()
Response.ContentType = "application/pdf"
Response.AddHeader("Content-Disposition", "attachment;filename=Combinado.pdf")
Response.BinaryWrite(ms.ToArray())
Response.End()
 

Gracias y un saludo.