Success!
Using the info from this post http://groups.google.com/group/facebooker/browse_thread/thread/abd2d0f91bfe8ac6
I got the following working:
<a href="<%= root_url(:canvas => false) -%>my_pdf.pdf">...image tag here...</a>
The only downside is that it downloads the file without comment, so it looks like nothing happened.