It's probably advisable to first add the PDF as a Baggage File. Then when you are creating the TOC entry point, you point at the Baggage File.
If that wasn't what you were hoping to see, you could also do it by simply linking to the PDF as you would a Web Page by specifying the explicit URL that would open the same file if you were doing it using a browser.
Helpful and Handy Links
Thank you! I'll give this a try.