Not yet available for Apple Products.
The website is available 100% offline. You can "install" the website to any Android or Apple device by following the instructions below.
Open your favourite web browser (I recommend Chrome for Android) and visit the Roll for Scenario website (http://www.rollforscenario.com/). Once there, click the 3 dots in the address bar as circled below. Then click Add to Home Screen. Fill in a name (or leave it as is) and press ADD. And that is all. On your home screen you will see the R4S icon. Simply click on this, even if you are not connected to the internet and it will open up and roll you another scenario.