Pay per view content is showing up in search results?

Hi, I have some PDF files stored in a folder on my hosting and there is a page on my site that lists them as links. This page is protected by pay per view. When you click on a link it displays the PDF as a new webpage. Pay per view seems to work just fine to protect access to the links of PDF's, however I noticed that if I search for the PDF directly in a seach engine, I can go straight to the webpage showing it without going through the pay per view setup, meaning people can access my content directly though a search engine result. How can this be fixed to protect my content?