code: function remove_doc_data(callback) {
    var folder = path.join(__dirname, '..');
    var doc_data_path = path.join(folder, 'doc_data.txt');
    fs.exists(doc_data_path, function (exists) {
      if (!exists) {
        return callback();
      }
      fs.unlink(doc_data_path, callback);
    });
  }
docs: pdftk creates a file called doc_data.txt during the burst split process.
  This file is not needed so remove it now