Both the demo (Lite) and full versions can only be downloaded from the Android/Apple/Windows App Stores.
Try out the Synthesis App demo (Lite) version:
In the Apple Itunes store for your iOS device - EN / FR / DE / IT / SP
In the Google Play store for your Android device - EN / FR / DE / IT
In the Microsoft store for your Microsoft device - DE
Buy the Synthesis App full version:
In the Apple Itunes store for your iOS device - EN / FR / DE / IT / SP
In the Google Play store for your Android device - EN / FR / DE / IT
In the Microsoft store for your Microsoft device - DE