How to make media library inaccessible to the the public

Hello there,

We are having some issues with the our WP site. We use lot of pdf files to display to general visitors. Now people have got expertise can see our folders by manipulating the URLs and might get access to our site which is SSL secured but we are still not sure if there is any possibilities to hack the site or steal any vital information or code from our site or not.

If the answer is yeas, how could we prevent that, by using any security plugin or tweaking in the core or what? please see attached screenshot along with the pdf file URL

we would like to get any kind of plugin which will protect our contents from unauthorized access esp from that including media files and also need to protect media contents from those users who have got limited access to our site like subscriber or editor level. is that possible?