All of the Quran is authentic.

 

The prophet (peace be upon him) preserved it in two methods.

 

His followers memorized it by heart one generation after another. We are now in generation number 29 of people who are certified by their teachers as they heard it from their teachers all the way to prophet Muhammad as he heard it from angel Gabriel as the angel heard it from God.

 

The second method, it was preserved on written documents like skins, leather, wood plates etc. After the prophet’s death by one year his caliph Abu Baker collected all these pieces together in one place. 15 years later the 3^rd caliph `Uthman wrote these documents of regular paper books that we inherit till now.

 

Almighty Allah knows best.