jQuery – Do something if checkbox is checked

const $pdfs = $('a[href$=".pdf"]');
$pdfs.on('click', function(event){
  //check if checkbox has been checked 
  //if zero checkboxes are checked 
  if ($(':checked').length === 0){
    //prevent download of document 
    event.preventDefault();
    //alert the user 
    alert('Please check the box to allow PDF downloads.');
  }
});