Dear Mark,
Lets say you have created 2 custom fields: pdf-thumbnail and pdf-file. The code you need in your View Template is:
<a href="[types field="pdf-file" raw="true"][/types]"> [types field="pdf-thumbnail"][/types] </a>
Please let me know if you are satisfied with my answer and if I can help you with any other questions you might have.
Regards,
Caridad