AutoCAD .NET :: Export PDF As Valid File Extension

I'm trying to make an application that would take a drawing(dwg) and convert it to pdf.  Since I can do it directly in autocad 2013, i presume that there should be a function in the API, that i could use to do the same.  So far, i tried with AcadDocument.export(filename, FileExtension, AcadSelectionSet) with no luck. I cannot put PDF as a valid file extension.

Here's my code

Public sub SaveToPDF(ByVal psfilename As String)

Dim oAutocad As AutoCAD.AcadApplication = New AutoCAD.AcadApplication()

Dim oAcadDoc As AutoCAD.AcadDocument = oAutocad.Documents.Open(psfilename, True)

Dim sFolder As String = psfilename.Substring(0, psfilename.LastIndexOf("") + 1)

[code]....