You can use this template for publishing your stories, docs, lessons in pdf format
to the play store. If you have the doc in other format (txt, docx etc.) you can convert it in pdf here
want to find niche for your app? Try making app for your social group/language. If you know several languages, use less popular one. Targeting small groups of people cam make your app a lot more popular. You'll be able to find niches that are free from huge players on app market and make a great app for your social group.