How to set hyperlinkfield to make it point to local file?

In Web.Config File

appSettings --Begin
add key="AboutTess_Pdf_Path" value="Documents/sampleAboutTess.pdf"
appSettings --End



In .CS File
if(!Page.IsPostBack)
{

string strPDFPath = ConfigurationSettings.AppSettings["AboutTess_Pdf_Path"].ToString();

string popupScript = "Javascript: return " +
"window.open('"+ strPDFPath + "', 'CustomPopUp', " +
"'width=600, height=600, menubar=no, resizable=yes');";

//Attach Script to button control
btnDownloadPDF.Attributes.Add("onclick",popupScript);
}