The easy answer is for you to google how to use Server.MapPath. First result is to MSDN,
Server.MapPath Method[
^]
Server.MapPath takes a virtual path. So, it would be something like
string template = Server.MapPath("~/MyServerFiles/fw.pdf");
Then template will end up having something like "c:\inetpub\wwwroot\mysite\MyServerFiles\fw.pdf"