Save pptx as pdf via excel

I am trying to convert all pptx files to PDF file transfer path.

my code:

Sub pptxtopdf()

    Dim ppt As Object
    Dim objFSO As Object
    Dim objFolder As Object
    Dim objFile As Object


    Dim i As Integer
    On Error Resume Next

    Set ppt = GetObject(, "PowerPoint.Application")
    If ppt Is Nothing Then
    Set ppt = CreateObject("PowerPoint.Application")
    End If
    On Error GoTo 0


    'Create an instance of the FileSystemObject
    Set objFSO = CreateObject("Scripting.FileSystemObject")
    'Get the folder object
    Set objFolder = objFSO.GetFolder("P:\Operations\Data & Deliverables\Projects\Amica\presentation_workspace\1_ spring 2015\Presentations\Volvo")
    i = 1
    'loops through each file in the directory 
    For Each objFile In objFolder.Files

        Set WDReport = ppt.Presentations.Open(objFile.Path)

        Dim FileName2 As String
        FileName2 = Replace(objFile.Path, "pptx", "pdf")

        'WDReport.ExportAsFixedFormat FileName2, ppFixedFormatTypePDF
        WDReport.SaveAs FileName2, ppSaveAsPDF

        WDReport.Close
        ppt.Quit

        Set ppt = Nothing
        Set WDReport = Nothing


        i = i + 1
    Next objFile


End Sub

      

        
        
        
      

    

error msg

Presentation.SaveAs :  Invalid enumeration value. 

      

        
        
        
      

    

Can't see what I am doing wrong?

same problem as here but the solution didn't work for me - Excel macro to save pptx as pdf; error with code