Please read my problem and sort out in c#

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ASP.NET

I have a download link button. on it i performing downloading of pdf file. its perfectly perform downloading. but if a linkbutton with in Gridview with same work not performing downloading.

Can u tell me why and solution of that problem.

try { 
string contentType = ""; 
//Get the physical path to the file. 
string invoice = "~/Invoice/" + DropDownList1.SelectedItem.ToString() + ".pdf"; 
string FilePath = HttpContext.Current.Server.MapPath(invoice); 
string fileExt = Path.GetExtension(invoice).Split('.')[1].ToLower(); 

if (fileExt == "pdf") 
{ 
 //Set the appropriate ContentType. 
 contentType = "Application/pdf"; 
} 

//Set the appropriate ContentType. 
Response.ContentType = contentType; 
Response.AppendHeader("content-disposition", "attachment; filename=" + (new FileInfo(invoice)).Name); 
//Write the file directly to the HTTP content output stream. 
Response.WriteFile(FilePath); 
Response.End(); 
} 
catch 
{ 
 //To Do 
}

Thanx in advance