jQuery toggle class and content on click of link
$('.hidden-content').hide(); can be moved to CSS. This will be better UX and user will not see any flickr on the screen if page take long time to load and thus hide the element by JavaScript.

.hidden-content {
    display: none;
}


To check if an element have a class, hasClass() can be used instead of is().

The methods can be chained $i.removeClass("angle-down").addClass("angle-up"); but there is better way to do this.

To toggle the class on an element, toggleClass() can be used. This also accepts callback function.

$('selector').toggleClass(function() {
    return $(this).children().hasClass('angle-down') ? 'angle-up' : 'angle-down';
});


toggleClass also accepts list of classes separated by space. If the element have already applied one of the class from it, toggleClass will remove it and add other classes. Now, the code will become,

$('selector').toggleClass('fa-angle-down fa-angle-up');


Demo:

Note that in Demo, I've added Font-Awesome to demonstrate the functionality.



$(document).ready(function() {
  $(".hidden-content-reveal").click(function() {
    $(this).children().toggleClass('fa-angle-down fa-angle-up');

    $(this).next('.hidden-content').slideToggle();
    return false;
  });
});
.hidden-content {
  display: none;
}
&lt;link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet" /&gt;
&lt;script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"&gt;&lt;/script&gt;
&lt;div&gt;
  &lt;p&gt;Lorem ipsum dolor sit amet, consectetur adipiscing elit.&lt;/p&gt;
  &lt;a href="#" class="hidden-content-reveal"&gt;
    &lt;i class="fa fa-angle-down" style="font-size:24px"&gt;&lt;/i&gt;
  &lt;/a&gt;
  &lt;div class="hidden-content"&gt;
    &lt;p&gt;Nulla facilisi. Cras non egestas lectus. Duis malesuada eleifend neque, eget tincidunt elit convallis sed.Nulla facilisi. Cras non egestas lectus.&lt;/p&gt;
  &lt;/div&gt;
&lt;/div&gt;