title: How to download file with asp.net on buttton's onClick event?
body: <p>I have a String variable (in C#) that contain the full path of PDF file on my server 
  (like that "~/doc/help.pdf").</p>
  
  <p>I want that in click on button, this file will download to the client computer. </p>
  
  <p>I created a button and made onClick event in C#. Now, which code should I write to do that?</p>