You can drop a web.config in that folder which will prevent the files from being accessed unless they are in a specific role:
<configuration>
<system.web>
<authorization>
<allow roles="WHATEVER-ALLOWED-ROLES"/>
<deny users="*"/>
</authorization>
</system.web>
</configuration>
If you only want to lock down that specific file you can wrap that <system.web> with <location path="filepath-and-name">
This will likely require you to add the following handler to your root web.config in the "handlers" section, as usually IIS will serve up the file before ASP.NET touches it. This will make PDFs go through ASP.NET which can then handle the Role restrictions from above:
<add name="PDFHandler-Integrated" path="*.pdf" verb="GET" type="System.Web.StaticFileHandler" modules="ManagedPipelineHandler" requireAccess="Script" preCondition="integratedMode" />