How to protect media upload content

I’m running a membership site on the latest version of WordPress, using WP-Members plugin.

I can protect pages and posts that are meant for members only but what about uploads via the media tool? I have a series of PDFs intended for the members only but have discovered that when I’m logged out I can still call up the PDF documents – they’re not blocked or protected.

How can I go about protecting them so they’re not able to be picked up by search engines or by visitors who might be wanting to see what they could find without logging in?