Allah educated Muhammad PBUH Himself. There is no-one who taught Muhammad PBUH in this temporary world. So he had no teacher and he was not able to write, that is why he is called an Ummi. Those who say that Muhammad PBUH invented the Quran and wrote it by himself, they should know that no-one ever taught him anything and he did not even write a single word during his lifetime. So how could Muhammad PBUH invent a book that is a masterpiece of the finest Arabic literature and that book is also a miraculous book? So the Quran is the word of God.