You can definitely use Form Guides to guide the user through based on a value chosen in a combo box.
A feature of Form Guide is the "answer-based navigation" where you can add questions, and based on their response either prompt with another question or display a new panel in the Form Guide.
Start up the Guide Builder, then just have play around with the answer based navigation by clicking on the "New Panel", click "Set Properties" (only if the form's objects are displayed in the right hand window), then click on the "Specify answer-based navigation" panel.
The added benefit is that you get a really cool looking UI (instead of the boring old PDF).
Have fun!
John.