how to combine pdf files into one file

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C#

VB

how to combine pdf files into one file, in a directory I have some pdf files that I want to combine into 1 file

I use the following code:

  Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim filePaths As String() = System.IO.Directory.GetFiles(path)
     
        For Each fileName As [String] In filePaths
            Dim targetFolder As String = path
            Dim fi As New System.IO.FileInfo(fileName)
            'fi.CopyTo(System.IO.Path.Combine(path, (fi.Name) & ".pdf"), True)
            Label1.Text = filePaths.Length
            alList.Add(fileName)
        Next
        CombineReport(alList, path)
    End Sub

    Public Shared Function CombineReport(ByVal pdfFiles As IList, ByVal pdfFile As String) As String
        Try
            Dim sb As New System.Text.StringBuilder()
            For Each o As Object In pdfFiles
                sb.Append(o.ToString())
                sb.Append(" ")
            Next
            sb.AppendFormat("cat output {0} allow printing dont_ask", pdfFile)
            System.Diagnostics.Process.Start("C:\WINDOWS\system32\pdftk", sb.ToString()).WaitForExit()
                        
            Return pdfFile
        Catch ex As Exception
            Throw ex
        End Try
    End Function
End Class

but the source code is not nothing changed, please help if anyone knows me out of this problem