I've just used this code snippet to save one of my own .pptx files as a pdf
Dim oPresentation As Presentation
Set oPresentation = ActivePresentation
Dim targetFolder As String, targetFile As String
targetFolder = oPresentation.Path
targetFile = Replace(oPresentation.Name, ".pptx", ".pdf")
oPresentation.ExportAsFixedFormat Path:=targetFolder & "\" & targetFile, FixedFormatType:=ppFixedFormatTypePDF