Set iframe contents using jQuery

HTML

<div id="container">
<iframe src="lesson2.pdf" id="iframe1"></iframe>
</div>
<button disabled class='submitBtn'> Submit</button>

CSS

#iframe1 {
  border: solid 1px;
  border-radius: 8px;
  color: red;
  padding-top: 1em;
  width: 100%;
  height: 500px;
}

#container {
  height: 200px;
  border: 1px solid black;
  overflow: scroll;
}

JavaScript

var containerEl = $("#container")[0];

$("#container").scroll(function () {
	if (containerEl.scrollTop + containerEl.clientHeight >= containerEl.scrollHeight) {
  $('.submitBtn').prop('disabled', false);
  }
});