I'm afraid you've wandered into a mostly unused forum.
Try posting your question in the InDesign forum here:
And when you do, please include any of the applicable information listed below:
Boilerplate text from Best Practices FAQ
Supply pertinent information for quicker answers
The more information you supply about your situation, the better equipped other community members will be to answer. Consider including the following in your question:
* Adobe product and version number
* Operating system and version number
* The full text of any error message(s)
* What you were doing when the problem occurred
* Screenshots of the problem
* Computer hardware, such as CPU; GPU; amount of RAM; etc.
You have to Export each page separately. Why the Export-to-PNG?
Export a PDF, open that in Photoshop and save.
The real question is why you need a PNG for a brochure.