File Protection :: Secure Files On My Server

I need to be able to secure files on my web server. I am using asp to secure access to links and pages, for example:

<%If Session("manager")=FALSE Then%>
You are not authorized to view this page
<%Else%>
<<<Page Code>>>
<%End If%>

The place I'm running into problems is with files. I have a lot of charts and such in PDF version. I kind of doubt there is a way to secure these files with asp, but I thought it would be worth a try.

My biggest issue is that PDFs are stored in the browser's history, so once the page has been accessed, anyone using the browser can get to thatunsecured PDF. As a brute force fix, is there some way to simply erase the site from the browser history? If not, is there a way to secure the PDF, or does someone know of a better group to post on?