A “mock up” is a non-working prototype of a product under development, is used to define the design and normally use it to pass to the customer before starting the product development phase.
MockApp, is a free Osx software downloadable from this web site where it is possibile to obtain a collection of files and images that will allow you to make a presentation in PowerPoint or Keynote presentation and export directly to PDF format.
Here are some videos on Youtube that shows its use:
MockApp Demo – Part 1
MockApp Demo – Part 2