The Quran is a miracle under the following three categories: miracle in terms of the words of the Quran, miracle in terms of the contents and miracle in terms of its bringer.
Neither anything has been decreased nor anything has been increased which was not present inside in the genuine manuscript of the Holy Quran.
Since Quran, according to the traditions, has an exterior and some inner aspects, its rules in regards to recite it has divided into the two rules: 1. Apparent rules 2. Inner (spiritual) rules.
The Bridge of Sirat:
Although the term the "Bridge of Sirat" hasn’t been used in the Quran, it has been mentioned in hadiths and traditions. For example Imam Sadiq (as) has mentioned this term while explaining the word "mirsad" in verse 14 of Surah Fajr, saying that it is a bridge that extends across Hell.
The Holy Quran was compiled in several stages. Imam Ali, peace be upon him, compiled the Quran on the basis of the chronological order of revelation but finally the compilation rendered by the caliphs became public and the Ahlul-Bayt (A.S.) also completely certified the present compilation.