Its a tad unique. You have to start from a technical standpoint. Since the files are pdf, the restriction on the file has to be serverbased. Normally, a login with viewing html,php,asp page is the case. And then the login restrictions can be simply coded into the webpage "if username, then etc". But with a pdf you can't do that. You can put a restriction on a folder on the server. A bit different than downloading a user login system and installing on your site.
I'd start with your host. Explain your requirements to them, and ask for their assistance with your server setup and developer as needed for programming. Erm, you are your own host. Are you capable of that? First question.