[Android]How to add an Expansion file to Android App

I m a new app developer and know the least about Android. I've developed a Notepad. I want to keep its installation size minimal and would like to add options like Fonts as an optional downloadable addon. I do not have any code or logic of how to make that happen but since I have seen this type of support in a few Android editing apps, so its definitely possible. I need a full tutorial to help me do that in my application. Kindly help me out of it. My Notepad is a simple Note making app based on the tutorials provided by Google and uses SQLite database, it is heavily focused on Themes and looks and have a very beautiful note sharing system. You can check the app here Easy Pad Link