Nginx: turn off gzip to PDF files

I am using this code to serve a file for download via PHP:

$file='file.pdf';
$filepath="/path-to-download-folder/$file";

if(!file_exists($filepath)){
  header('HTTP/1.1 404 Not Found');
  exit;
}elseif(!is_file($filepath) or !is_readable($filepath)){
  header('HTTP/1.1 403 Forbidden');
  exit;
}else{
    header('Cache-Control: no-store, no-cache, must-revalidate');
    header('Content-Type: '.mime_content_type($filepath));
    header('Content-Length: '.filesize($filepath));
    header('Content-Disposition: attachment; filename="'.$file.'"');

    set_time_limit(0); // Big files/slow connections may result in incomplete downloads
    readfile($filepath);
    exit;
}

However some people are receiving a corrupted PDF file.

For example: http://mlkshk.com/r/8FGS

In Nginx I have this:

    gzip on;
    gzip_disable "msie6";
    # gzip_vary on;
    # gzip_proxied any;
    # gzip_comp_level 6;
    # gzip_buffers 16 8k;
    # gzip_http_version 1.1;
    # gzip_types text/plain text/css application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript;

My question is: how to set it not to gzip PDF files? Maybe this is the problem...

Thanks.