Recently I had a client who was looking for a text onclick pdf to pop up like an image rather than the ugly page that usually displays with pdfs. I found this plugin: Easy Fancybox.
Overall, I would give this plugin 5 stars.
**Extra note: If you need a popup to display upon people visiting your site instead of onclick, try WordPress PopUp. It has a nice number of options and is easy to set up.
Click here to see it in action:
Celebrate New Year’s Eve in Style
Note for mobile devices: add this script in the footer (or text widget in the footer) to stop fancybox from working on mobile devices. I find that it doesn’t work well there.
Note: One of my favorite plugins, WP Supersize, causes a conflict with this plugin, so if you are having problems, try disabling it.