app/views/hyrax/file_sets/media_display/_pdf.html.erb
<% if Hyrax.config.display_media_download_link? %>
<div>
<h2 class="sr-only"><%= t('hyrax.file_set.show.downloadable_content.heading') %></h2>
</div>
<iframe id="pdf-viewer" width="100%" height="600px" frameborder="0"
scrolling="no" marginheight="0" marginwidth="0"
title="PDF Document" src="<%= viewer_url(download_path(file_set)) %>"></iframe>
<% else %>
<div>
<%= image_tag thumbnail_url(file_set),
class: "representative-media",
alt: "",
role: "presentation" %>
</div>
<% end %>