Downloading Pdf From Server

i want download some pdf from server...i write folllowing code..

Response.ContentType = "application/pdf";
StringBuilder text1 = new StringBuilder();
text1.AppendFormat("attachment;filename={0}", filename);
Response.AddHeader("content-disposition", text1.ToString());
StringBuilder test = new StringBuilder();
test.AppendFormat("~/certificates//data//{0}", filename);
FileStream sourceFile = new FileStream(test.ToString(), FileMode.Open);
long FileSize;
FileSize = sourceFile.Length;
byte[] getContent = new byte[(int)FileSize];
sourceFile.Read(getContent, 0, (int)sourceFile.Length);
sourceFile.Close();
Response.BinaryWrite(getContent);
Response.Flush();
Response.End();

but when i used in sever it not working. its only path related issue. how get half path of server dir......like i read Request.MapPath().....but i not understand