In this video I demonstrate how to build a custom command that will open up a PDF document directly from a button on the ribbon interface. Please note that you should always create a partial CUI file to store your customizations in. Also, I want to credit Sam Lucido for setting me on the right path and you can find a link to that video here: https://knowledge.autodesk.com/support/autocad/learn-explore/caas/screencast/Main/Details/f2f171de-f7ba-4023-bfcc-95bcaa66ce13.html
Source: Autodesk
