With other uploaders in rails, it's not super straightforward to convert a PDF to an image. But, by using Dragonfly's ImageMagick plugin, it's real simple.
Make sure you're using the ImageMagick plugin.
Dragonfly.app.configure do plugin :imagemagick end
And you'll want GhostScript installed on your machine.
The markup for it, though is simple. If you want a small,
200x200 thumbnail of a pdf, it's as simple as the following.
<%= image.image.thumb('200x200#', :format => 'png', :frame => 0).url %>
Note: This assumes you have an
image object that has an uploader mounted to an
image accessor. You'll want to update to your application.