The file cannot be below the directory that is mapped as the root directory for the root application on the webserver. Example:
If you have an application on the sever at:
C:\Inetpub\wwwroot\MyApplication\
And that application is mapped to the domain:
http://www.myapplication.com/
Then you cannot link to any files below C:\Inetpub\wwwroot\MyApplication\.
The simple solution here is to copy these files to the root directory for the root application, or a sub directory therein.
If that is not an option then your best bet would be to create a generic handler page (a .ashx page), that can do a binary read/write of a specified file anywhere on the server, and change the mime-type of file (by setting the Response.ContentType) to "application/pdf."
-- Dan