Issue Opening File in ASP.net

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C#

ASP.NET

Microsoft JScript runtime error: Sys.WebForms.PageRequestManagerParserErrorException: The message received from the server could not be parsed. Common causes for this error are when the response is modified by calls to Response.Write(), response filters, HttpModules, or server trace is enabled.

Details: Error parsing near '%PDF-1.2
%??
1 0 ob'

I am getting this above error when trying to open file in asp.net

Code which i am using

string ViewFileName = Server.MapPath(DocumentPath + lblHiddenFileName.Text);
 FileInfo infofile = new FileInfo(ViewFileName);
 string myFile = ViewFileName;
 WebClient myWeb = new WebClient();
 Byte[] myBuff = myWeb.DownloadData(myFile);
 Response.Expires = 0;
 Response.CacheControl = "no-cache";
 Response.AppendHeader("Pragma", "no-cache");
 Response.Buffer = true;
 Response.ContentType = contenttype;//"application/pdf";
  Response.AddHeader("Content-Disposition", "attachment; filename=" + infofile.Name);
 Response.BinaryWrite(myBuff);