SecurityAssist on its own can't protect digital assets such as PDFs and images. You could combine SecurityAssist with DigitalFilePro and accomplish it.
Use SecurityAssist to manage the log in and then follow the steps to setting up Digital Downloads (with DigitalFilePro) to protect the location ect of the digital assets.
Here's the link to how to accomplish the piece with DigitalFilePro
A way to handle it very simply would be to wrap the links to the digital assets with IF THEN logic and check to see if the person is logged in. If they are logged in then show the links / If they're not logged in do not show the links. This wouldn't require DigitalFilePro, but the location of the digital assets would not be protected.
Hope that helps you or at least points you in the right direction.