Well in 2 lines of code you can do that assuming you have acrobat reader installed in your computer. System.Diagnostics namespace has a process class to do this.
add
using System.Diagnostics;
Place this code to your button click event.
string pdfFileName = "C:\\mypdfFileName.pdf";
System.Diagnostics.Process.Start(pdfFileName);
c#.net;
Response.Write("<SCRIPT language=javascript>var pdf=window.open('" +
result + "','PDF');pdf.moveTo(0,0);</SCRIPT>");
or
<script type="text/javascript">
function openWindow(path)
{
var myWin = window.open(' " + pdfFileName + " ','mywindow','width=400,height=200');
}
</script>
...
<input type="button" onClick="openWindow(varName)">