Linking to a PDF is done the same way as linking to any other document or file. If you're getting a 404, you probably have the path to the link wrong or their isn't a MIME type for PDF on the server.
You can add mime types using the web.config file. Open the web.config file and find the staticContent section. In this section, you should see several mime types. Add the following with the other mime types inside of the staticContent section:
<remove fileExtension=".pdf" />
<mimeMap fileExtension=".pdf" mimeType="application/pdf" />