If you want to add a button to a PDF to perform the Export PDF Action.
You would need create a button to perform the following action.
You could go to tools and select rich media where in you would find that add button option.
Place the button at desired location and then click on button properties just on bottom left of the button.
You could play it with as per yourself to set the desired conditions.
The other option of exporting PDF is directly through Acrobat by selecting option Export to a certain file format.