use security assist to create the login/ registration system first.
since the site will have an admin and regular users, you will want to use User Level authentication.
there is a tutorial for user level authentication in the Archived Documentation section of the Security assist support page:
use Data Bridge to create the pages for the admin to manage the files table and upload the PDF. The upload will use the File Manipulation -> File Upload behavior.
use the File Download behavior to allow the users to download the pdf.