Add a PDF or hyperlink it

I had the same question, this is what I found to work for me, publishing to CourseMill.

Create and "extra" chapter at the end of the course, containing one page called "Quick Reference"

- On show of the page, Launch a Program/Document, the Program/Document being "Quick Reference.pdf"

On the original page where the user gains access to the .pdf put a button

- on click, go to Chapter/Section/Page - Name: Quick Reference. Check the Open in New Window box.