I feel like the answer is yes, thanks to permissions/role settings, etc, but I need to be certain that there is a way to prevent the site from being indexed, and prevent files from being accessible without login with approved user role. 

Can DNN be used to create a secure, password-protected portal? I want only the login screen and it's assets/images, etc, to be accessible/viewable by all, nothing else.

Lets say someone types in Mysite.com/thisisa.pdf

 Is there a way to force either A) redirect to login page or B) 404 or some not found, or not accessible page?

I have googled extensively and have not found precisely the answers I'm looking for.  If someone can point me in the direction to creating a secure portal, that would be awesome.

Thank you, I've been toying with that.  I see that existing Standard folders can't be changed to Secure or Database folders, is there another way?