Membership Pro - secure pages but what about linked docs?

So - wondering about how to secure private documents - while I can protect a page with your membership pro plugin - how can I protect pdf and other 'private' media from being exposed to public search and hackers etc. I understand that I can disallow specific directories in htaccess, possibly encrypt each file separately, and use a 'no follow' tag in html - though even Google admits items still might get past all of that - but are there any bullet proof ways to block access from the outside without disrupting the user experience?