I have spent half a day to find out how to get to the object of pdfforge.
Lastly, I have found that the answer is in the tab "Project->Add Reference…->COM->PDFCreator which the path point to “C:\Program Files\PDFCreator\PDFCreator.exe” (I wonder that COM object can be .exe file, I have to read more about COM… : - )
Have fun with this Library
Kang