This .NET class wraps the ObjectARX class AcDbPdfDefinition.

Autodesk.AutoCAD.DatabaseServices.DBObject
    Autodesk.AutoCAD.DatabaseServices.UnderlayDefinition
        Autodesk.AutoCAD.DatabaseServices.PdfDefinition

Public Class PdfDefinition
Inherits UnderlayDefinition

public class PdfDefinition : UnderlayDefinition;