Private Sub Button2_Click(sender As System.Object, e As System.EventArgs) Handles Button2.Click

        Dim FileName As String = "c:\prova.pdf"
        Crea_PDF_File(FileName)

        Dim TempFileName As String = "c:\temp_demo.pdf"
        Dim PasswordVecchia As String = "mypass"
        Dim PasswordNuova As String = "1234"


        'Verifica se il file esiste
        If System.IO.File.Exists(FileName) Then
            'Verifica se il file è protetto da password
            If IsPasswordProtected(FileName) Then
                'Verifica prima se la password è corretta
                If IsPasswordProtectedCorrect(FileName, PasswordVecchia) Then
                    'Modifica la password al file
                    ChangePasswordPDF(FileName, TempFileName, PasswordVecchia, PasswordNuova)
                Else
                    'restituisce il messaggio d'errore che la password è sbagliata
                    MsgBox("Errore la Password inserita non è corretta", vbCritical, "ATTENZIONE!!")
                End If
            Else
                'Il file non è protetto da password 
                CreatePDFPassword(FileName, TempFileName, PasswordNuova)
            End If
        Else
            MsgBox("File non esistente", vbCritical, "ATTENZIONE!!")
        End If
    End Sub

    ''' <summary>
    ''' Cambia password al file PDF 
    ''' </summary>
    ''' <param name="sourcePdfPath">Percorso del file sorgente PDF (ex. c:\demo.pdf)</param>
    ''' <param name="outputPdfPath">Percorso dove salvare il nuovo file PDF (ex. c:\test.pdf)</param>
    ''' <param name="PasswordNew">Password Nuova</param>
    ''' <remarks></remarks>
    Private Sub CreatePDFPassword(ByVal sourcePdfPath As String, ByVal outputPdfPath As String, ByVal PasswordNew As String)
        Try
            Dim pageCount As Integer = 0
            Dim currentPage As Integer = 0
            Dim pdfDoc As iTextSharp.text.Document = Nothing
            Dim writer As iTextSharp.text.pdf.PdfCopy = Nothing
            Dim page As iTextSharp.text.pdf.PdfImportedPage = Nothing
            Dim currentPDF As Integer = 0

            Dim pdfReader As New PdfReader(sourcePdfPath)

            pdfDoc = New iTextSharp.text.Document(pdfReader.GetPageSizeWithRotation(1))
            writer = New iTextSharp.text.pdf.PdfCopy(pdfDoc, _
                                                  New IO.FileStream(outputPdfPath, _
                                                  IO.FileMode.OpenOrCreate, _
                                                  IO.FileAccess.Write))

            writer.SetEncryption(PdfWriter.STRENGTH128BITS, PasswordNew, PasswordNew, PdfWriter.AllowCopy)

            pageCount = pdfReader.NumberOfPages
            pdfDoc.Open()

            While currentPage < pageCount
                currentPage += 1
                pdfDoc.SetPageSize(pdfReader.GetPageSizeWithRotation(currentPage))
                pdfDoc.NewPage()
                page = writer.GetImportedPage(pdfReader, currentPage)
                writer.AddPage(page)
            End While

            pdfDoc.Close()

        Catch ex As Exception
            MessageBox.Show(ex.Message)
        End Try
    End Sub

 'Crea un nuovo file PDF con la password
    Private Sub Crea_PDF_FileEncryptPassword(ByVal PercorsoFile As String, ByVal PasswordUser As String, Optional ByVal PasswordOwner As String = Nothing)
        Dim pdfWrite As PdfWriter
        Dim pdfDoc As New Document()
        Dim pdfMemoryStream As New IO.FileStream(PercorsoFile, IO.FileMode.Create)
        pdfWrite = PdfWriter.GetInstance(pdfDoc, pdfMemoryStream)
        'Crea il file con la password
        pdfWrite.SetEncryption(PdfWriter.STRENGTH128BITS, PasswordUser, PasswordOwner, PdfWriter.AllowCopy)
        pdfDoc.Open()
        pdfDoc.Add(New Paragraph("Hello World"))
        pdfDoc.NewPage()
        pdfDoc.Add(New Paragraph("Hello World Again"))
        pdfDoc.Close()
    End Sub
    ''' <summary>
    ''' Cambia password al file PDF 
    ''' </summary>
    ''' <param name="sourcePdfPath">Percorso del file sorgente PDF (ex. c:\demo.pdf)</param>
    ''' <param name="outputPdfPath">Percorso dove salvare il nuovo file PDF (ex. c:\test.pdf)</param>
    ''' <param name="password">Password Attuale</param>
    ''' <param name="PasswordNew">Password Nuova</param>
    ''' <remarks></remarks>
    Private Sub ChangePasswordPDF(ByVal sourcePdfPath As String, ByVal outputPdfPath As String, ByVal password As String, ByVal PasswordNew As String)
        Try
            Dim pageCount As Integer = 0
            Dim currentPage As Integer = 0
            Dim pdfDoc As iTextSharp.text.Document = Nothing
            Dim writer As iTextSharp.text.pdf.PdfCopy = Nothing
            Dim page As iTextSharp.text.pdf.PdfImportedPage = Nothing
            Dim currentPDF As Integer = 0

            Dim pdfReader As New PdfReader(sourcePdfPath, New System.Text.ASCIIEncoding().GetBytes(password))

            pdfDoc = New iTextSharp.text.Document(pdfReader.GetPageSizeWithRotation(1))
            writer = New iTextSharp.text.pdf.PdfCopy(pdfDoc, _
                                                  New IO.FileStream(outputPdfPath, _
                                                  IO.FileMode.OpenOrCreate, _
                                                  IO.FileAccess.Write))

            writer.SetEncryption(PdfWriter.STRENGTH128BITS, PasswordNew, PasswordNew, PdfWriter.AllowCopy)

            pageCount = pdfReader.NumberOfPages
            pdfDoc.Open()

            While currentPage < pageCount
                currentPage += 1
                pdfDoc.SetPageSize(pdfReader.GetPageSizeWithRotation(currentPage))
                pdfDoc.NewPage()
                page = writer.GetImportedPage(pdfReader, currentPage)
                writer.AddPage(page)
            End While

            pdfDoc.Close()

        Catch ex As Exception
            MessageBox.Show(ex.Message)
        End Try
    End Sub
    'Verifica se il file è protetto da password
    Private Function IsPasswordProtected(ByVal pdfFullname As String) As Boolean
        Try
            Dim pdfReader As New PdfReader(pdfFullname)
            Return False
        Catch generatedExceptionName As BadPasswordException
            Return True
        End Try
    End Function

    'Verifica se la password è corretta per aprire il file
    Private Function IsPasswordProtectedCorrect(ByVal pdfFullname As String, ByVal Passwords As String) As Boolean
        Try
            Dim pdfReader As New PdfReader(pdfFullname, New System.Text.ASCIIEncoding().GetBytes(Passwords))
            Return True
        Catch generatedExceptionName As BadPasswordException
            Return False
        End Try
    End Function