This project holds a demo application for the combine_pdf gem, so you can learn how to use the code, copy and adjust for your needs.
The project uses a simplified implementation of The Plezi framework rather than using Rails. This should allow to easily read the code for the actual PDF handling, in the controller class.
Although using Plezi's websocket features to upload and download PDF data would have been a better approach, a simplified Http based approach was favored, so that both the form and the controller could be easily copied over to Rails applications with only minor adjustments.
You can see the demo at work (it's hosted on Heroku). I know the design is less than beautiful, feel free to contribute a nicer design, if you wish.