problem with downloading some pdf's

<> I've gotten this one report of having a problem downloading a PDF: "I have tried twice, both times its starts to download but stops about half way or three-quarters the way complete. I get a error message that the download was not completed. When I go to look at the message there is just numbers. I have tried to download again but the "numbers" show up and it won't let me try again." This only happens for this person with one file - the rest of the files work fine. I am streaming the file with this code: // will return FALSE if the file is not found // will not return anything if the file is found because the headers should // have already been sent. function streamfile($file) { if (file_exists($file)) {

       $extension = strtolower(stristr($file, "."));
             if ($extension == ".php" || $extension == ".html" || $extension == ".htm" || $extension == ".shtml") {
           include ($file);
       }
       else if ($extension == ".txt")
       {
           ?>
           <html>
           <body>
           <pre>
           <? include($file);?>
           </pre>
           </body>
           </html>
           <?
       }
       else {

           // a switch would be perfect here,
           // but it just didn't work last time...it was always
           // taking the 'default:' case
           if         ($extension == ".doc"){$type = "application/msword";}
           else if ($extension == ".pdf"){$type = "application/pdf";}
           else if ($extension == ".exe"){$type = "application/octet-stream";}
           else if ($extension == ".ppt"){$type = "application/vnd.ms-powerpoint";}
           else if ($extension == ".xls"){$type = "application/vnd.ms-excel";}
           else if ($extension == ".xml"){$type = "text/xml";}
           else if ($extension == ".zip"){$type = "application/zip";}
           else if ($extension == ".txt"){$type = "text/plain";}
           else {
               $type="application/octet-stream";
           }

           header("Content-Type: " . $type);
           header("Accept-Ranges: bytes");
           header("Content-Length: ".filesize($file));
           header("Content-Disposition: attachment; filename=".basename($file).";");                      //readfile($file);                                            $fp = fopen($file, 'rb');
           $buffer = fpassthru($fp);
           fclose($fp);
           exit();                            }                                  return TRUE;

} else {

       return FALSE;
   }           }

Anyone have any ideas? It would be appreciated. Scott