ability to remove metadata from uploaded files

I agree with @solideogloria.

Here is a good article how to remove the metadata in PDFs.
Removing metadata from PDF files using Exiftool and qpdf

I am actually using this in file_pre_save hook

function xxx_file_presave($file) {
  // only proceed with pdf
  if ( !isset($file->filemime) || !preg_match('/pdf/', $file->filemime) ) {
    return;
  }

  $tmpfilename = 'private://' . drupal_random_key();
  $file_realpath = drupal_realpath($file->uri);
  $tmpfilename_realpath = drupal_realpath($tmpfilename);
  try {
    copy($file_realpath, $tmpfilename_realpath);
    $a = `exiftool -all:all= $tmpfilename_realpath`;
    $b = `qpdf --linearize $tmpfilename_realpath ${tmpfilename_realpath}_linear`;
    $c = `exiftool -all:all= ${tmpfilename_realpath}_linear`;
    $d = `qpdf --linearize ${tmpfilename_realpath}_linear ${tmpfilename_realpath}_linear_cleaned_linear`;
    copy("${tmpfilename_realpath}_linear_cleaned_linear", $file_realpath);

    drupal_set_message(t('PDF metadata cleaned for @filename.', array('@filename' => $file->filename)));
  } catch (\Exception $e) {
    watchdog(WATCHDOG_ERROR, 'PDF metadata cleaning failed for @filename.', array('@filename' => $file->filename));
    watchdog(WATCHDOG_DEBUG, $e->getMessage());
  }
}