How do I create a custom Drupal 7 login?
Drupal's administrative permissions are not necessarily linked to any specific role, and individual permissions can define access to anything from viewing individual content types to administrative functions like creating and editing--they aren't all tied to administrative features. 

While it is true that all authenticated users will inherit permissions from Authenticated User, you can organize your permissions like so:


Anonymous User: (Permissions for users that don't need to log in.)
Authenticated User: (Remove all permissions except those needed to view documents)
Administrator (new role): (Permissions necessary to administer the site)


Then, give the Administrator role only to the accounts used to administer the site. Nothing says you have to give that role to every Authenticated User. Administrators will be able to view documents (as with any other authenticated user), though this sounds perfectly reasonable if I understand your use case.

Additional contributed modules allow for more fine-tuning over access control. For instance, the Field Permissions module may be useful if the files you're looking to distribute have been attached to nodes via File fields; this will allow you to determine access control at the field level if necessary.