I know, these theory of missing parts in Quran is from Shiate community; that's the reason No Hafiz (who memories the whole Koran by Heart) among them...If one have real faith and belief in Allah, messengers, angels, jinns & Koran then only one could become Hafiz otherwise one could not..it is one of the miracle of Quran.

In Quran Allah claims that He is the One who protects Quran from any manipulations and amendments as it is been done with earlier holy books given to Jews and Christians i.e. Tohrat and Bible.