Dim fdfWriter As New FdfWriter()
Dim memStream As New System.IO.MemoryStream
Dim reader As PdfReader = New PdfReader("c:test.pdf")
Dim stamper As PdfStamper = New PdfStamper(reader, memStream)
Dim fields As iTextSharp.text.pdf.AcroFields = stamper.AcroFields

fields.SetField("cisNameClient", "TEST")
fields.SetFieldProperty("btnSubmit", "setfflags", PdfFormField.FLAGS_HIDDEN, Nothing)
fields.ExportAsFdf(fdfWriter)
fdfWriter.WriteTo(context.Response.OutputStream)