IIS is keeping hold of my generated files

byte[] asciiBytes = getPdf(...);
try{
BinaryWriter bw = new BinaryWriter(File.Create(filename));
bw.Write(pdfBytes);
}
finally {
  if(null != bw)
    bw.Close();
}

Response.ContentType = "application/pdf";
Response.AppendHeader("Content-disposition", "attachment; filename=" + PDID + ".pdf");
Response.WriteFile(filename);
Response.Flush();