function getFolderByName_(folderName) {

  // Gets the Drive Folder of where the current spreadsheet is located.
  const ssId = SpreadsheetApp.getActiveSpreadsheet().getId();
  const parentFolder = DriveApp.getFileById(ssId).getParents().next();

  // Iterates the subfolders to check if the PDF folder already exists.
  const subFolders = parentFolder.getFolders();
  while (subFolders.hasNext()) {
    let folder = subFolders.next();

    // Returns the existing folder if found.
    if (folder.getName() === folderName) {
      return folder;
    }
  }
  // Creates a new folder if one does not already exist.
  return parentFolder.createFolder(folderName)
    .setDescription(`Created by ${APP_TITLE} application to store PDF output files`);
}
Returns a Google Drive folder in the same location 
in Drive where the spreadsheet is located. First, it checks if the folder
already exists and returns that folder. If the folder doesn't already
exist, the script creates a new one. The folder's name is set by the
"OUTPUT_FOLDER_NAME" variable from the Code.gs file.

@param {string} folderName - Name of the Drive folder. 
@return {object} Google Drive Folder