code: function get_pdfs_in_directory(directory_path, callback) {
    var file_paths = [];
    var files = null;
    var walker = walk.walk(directory_path, { followLinks: false});
    walker.on('file', function(root, stat, next) {
      if (stat.name.match(/\.pdf$/i)) {
        var file_path = path.join(directory_path, stat.name);
        file_paths.push({file_path: file_path, file_name: stat.name});
      }
      next();
    });
  
  
    walker.on('end', function() {
      file_paths.sort(function (a,b) {
        if (a.file_name < b.file_name) {
          return -1;
        }
        if (a.file_name == b.file_name) {
          return 0;
        }
        return 1;
      });
      var output = {
        folder: directory_path,
        files: file_paths
      }
      return callback(null, output);
    });
  }
docs: Non-recursive find of all the files in a given directory that end with *.pdf
  @return {Object} output an object with the fields "folder" and "files" set
  files is an array of the absolute paths to the single page pdf files.
  
  Each entry in this array is an object with fields
  <file_name> and <file_path> set
  
  @return callback(<maybe error>, output)