Digital Textbooks

Hi:)
I‘ve been learning Japanese for a while now and was wondering if anyone can recommend any websites, where you can buy Japanese digital textbooks (pdf or ebooks). I mainly want to continue practising/learning on my tablet since I can just easily erase everything and redo the exercises. I‘ve looked online if I could find anything, but kind of hit a dead end. I am mainly looking for grammar exercise books and I‘d be thankful for any suggestions 🙂