Comments by Chetan Patel (Top 2 by date)

Use HTTPHandler to validate the user. where you can write the pdf file as response for valid users and also you can pass message for invalid user's. as you can set the type of HTTP response.
You just need to add one Handler class to handle the request. then configure it to web.config ex://

<httphandlers>
<add type="<namespace>.<classname>,<Assemblyname>" path="*.pdf" verb="GET">