ช่วยหน่อยครับ Error ตรงที่ Dim pdfReader = New PdfReader(existingFileStream)

Imports System.IO
Imports iTextSharp.text.pdf

Namespace PDFTest
    Public Class PdfTest
        Public Shared Sub ReplacePdfForm()
            Dim fileNameExisting As String = "C:\path\to\existing.pdf"
            Dim fileNameNew As String = "C:\path\to\new.pdf"

            Using existingFileStream = New FileStream(fileNameExisting, FileMode.Open)
                Using newFileStream = New FileStream(fileNameNew, FileMode.Create)
                    ' Open existing PDF  
                    Dim pdfReader = New PdfReader(existingFileStream)

                    ' PdfStamper, which will create  
                    Dim stamper = New PdfStamper(pdfReader, newFileStream)

                    Dim form = stamper.AcroFields
                    Dim fieldKeys = form.Fields.Keys

                    For Each fieldKey As String In fieldKeys
                        form.SetField(fieldKey, "REPLACED!")
                    Next

                    ' "Flatten" the form so it wont be editable/usable anymore  
                    stamper.FormFlattening = True

                    ' You can also specify fields to be flattened, which  
                    ' leaves the rest of the form still be editable/usable  
                    stamper.PartialFormFlattening("field1")

                    stamper.Close()
                    pdfReader.Close()
                End Using
            End Using
        End Sub
    End Class
End Namespace