How could the Quran came here on earth? And who teach them to read the Quran.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The Quran was revealed from Allaah to the Prophet through the angel Jibreel (Gabriel) . Allaah Says (what means): {The Trustworthy Spirit [i.e. Jibreel] has brought it down. Upon your heart [O Muhammad ] - that you may be of the warners. In a clear Arabic language.}[Quran 26:193-195]. Allaah further Says (what means): {Say, [O Muhammad ], "The Pure Spirit [i.e. Gabriel] has brought it down from your Lord in truth."}[Quran 16:102]. So, first it is the Prophet upon whom the Quran  was revealed by the angel Jibreel, he learnt it and then he taught it to the companions who in turn taught it to those who came after them, and so it was transmitted generation after generation until today, and it was transmitted without any falsification or distortion, Allaah Says (what means): {Indeed, it is We who sent down the message [i.e. the Quran] and indeed, We will be its guardian.}[Quran 15:9].

For more benefit please refer to Fatwa 88584 entitled: 'How could the Prophet read since he was illiterate.'

Allaah Knows best.