This .NET class wraps the ObjectARX class AcDbPdfReference.

Autodesk.AutoCAD.DatabaseServices.DBObject
    Autodesk.AutoCAD.DatabaseServices.Entity
        Autodesk.AutoCAD.DatabaseServices.UnderlayReference
            Autodesk.AutoCAD.DatabaseServices.PdfReference

Public Class PdfReference
Inherits UnderlayReference

public class PdfReference : UnderlayReference;