Where to procure E-books (PDF ver.)?

Hi, I'm a novice at getting E-Books for reading. I'm aware that Google Play Store has E-Books, there are Kindle Tablet, etc.

​

I intend to get a Android/Windows tablet for the purpose of reading E-books, News articles from news apps. I didn't consider any Apple Ipad, or devices as my Tech Ecosystem is just Android n Windows devices (Samsung, Asus).

​

I would like to have some advices and tips about how to procure E-Books from certified websites, links, etc. Thank you in advance. Sensei, Shi Fu 师傅😊😊