At first we must differentiate between the seven methods of recitation of the Quran, and the concept of the descending of the Quran in seven letters or styles (aḥruf). The first is called al-qirā'āt al-sab', and the second is called nuzūl al-Qurān 'ala sab' at ahruf.
In regards to the concept of the Quran coming down in seven letters, the Shi'a view of this is that it is a myth and cannot be accepted.
As for what does the word ḥarf here mean, Sunni scholars have mentioned up to forty views for this.
As for the Quran being preserved, this is the strong opinion of Shia Ulama.
Please read this information, from al-Bayan, by the late A.U. Seyid Khoei.
There is big difference between Qira'aat and Ahruf. Qira'aat are ways of reciting the same words of Quran according to the axon of different Arab tribes. Qira'aat keep the meaning same with slight different way of pronouncing the words .
Ahruf (plural of Harf) means different wordings. This claim of Ahruf was condemned by Ahlul Bayt (AS) who clearly said that Quran was sent from One Allah, on One Prophet, through One Angle and on one word not more.
From al-Fudhail Ibn Yasaar said: I told Abi Abdillah (Al-Sadiq A.S.): People say that Quran was revealed on seven Ahruf (words). He said: They are liars, enemies f Allah. Quran was revealed on One Word from The One . (Kitab Al-Kaafi).
We do not believe in more than one Harf of Quran which means one wordings not more. Different wordings means different meanings which is false claim.
There is no difference in wordings at all between Qira’aat.