I found the code in the following post: http://wp-types.com/forums/topic/attaching-a-link-of-a-pdf-to-a-featured-image/
<a href="[types field="pdf-file" raw="true"][/types]">
[types field="pdf-thumbnail"][/types]
</a>
Right now, it opens the PDF in the browser. That's fine, but is there a way to make it download instead?