The distortion of the Quran has a broad meaning that covers all possible changes, whether it’s the adding of words, omitting of words, changing the combination of the words or changing the combination of different phrases. Muslim Scholars have mentioned several reasons that prove the Quran has not been distorted; we will explain some of the logical reasons:
1- Since the Quran was revealed it's unique qualities like its astonishing order, the harmony between its verses, prophecies of future events, etc. The Quran we know today has all of these qualities and no one to this day has been able to bring a chapter similar to that of the Quran's in terms of it's unique and marvelous qualities. Therefore the Quran we read today is the same as the one revealed 14 centuries ago.
2- In order for a religion to be the Seal of religions it must have two features, first it should be a comprehensive religion (unlike other religions that are not the last of religions) and second it should be protected from all forms of distortion and falsification, otherwise our intellect would not accept it to be the Seal of Religions. Knowing that Islam is the Seal of Religions and that the Quran is the book of this religion (so the Quran is the book of the seal of religions) consequently the book of Islam that consists of most of its rulings cannot be distorted across time as well.
3- Muslims have always been connected with the Quran and have recited it in their prayers, mosques, and warfronts and have used its verses as arguments to prove that Islam is the true religion, in other words they have never put the Quran aside. Due to this fact there has been no period of history in which the Quran could have been altered without all Muslims finding out.
4- Due to the fact that during the time of the Prophet there were many believers who strived to memorize the Quran and recite it by heart, there could not have been a distortion of the Quran. There were so many of those who taught, recited and memorized the Quran that 70 of them died on only one incident named "Bi’r Ma’unah". From the time the prophet passed away to this day there has always been a mass number of Muslims that taught, recited and memorized the Quran. Therefore there is no possibility that the Quran could have been changed throughout the ages.
5- Historians have mentioned up to 43 of the prophet’s companions that were the " Transcribers of Revelation". Such a book could not have been changed due to the great of number of writers.
6- Since the advent of Islam and the revelation of the Quran the imams have encouraged Muslims to recite, ponder and act upon the Quran we have today. This proves that the Quran has remained untouched through the last 14 centuries, especially the first few centuries after Islam’s birth.
 Safinatul-Bihar, vol. 1 pg. 57.
The distortion of Religious texts and scriptures has been common among most religions and one of the main reasons new prophets were sent to people was to provide a corrective for the previous altered religions. But Islam, as the Seal of Religions, has been protected from any potential distortion and falsification.
This question can be answered in different ways depending on the individual who has posed the question. It can be answered from a Muslim perspective (using Quranic verses and traditions) and a non-Muslim one (using intellectual and logical argumentation and reasoning). Here we will answer the question from a non-Muslim perspective and won't touch on the many Quranic verses and traditions from the infallibles that clearly prove its immunity.
First: The Quran's Eejaz (Miracle)
Allameh Tabatabei argues that the Quran cannot have undergone distortion due to its I’jaz (its miraculousness). In this respect he says: "It is a historical fact that 14 centuries ago a Prophet named Muhammad arose and brought a book named the Quran that was his miracle for all times. The Quran was revealed with certain qualities that made it significant among other books and challenged all Arabs to bring anything like it. It is also historically certain that none of the prophet's adversaries were able to bring a chapter, even the size of the smallest chapters, that would resemble the Quran with its special qualities.
The current Quran that we have still has those qualities like its beautiful rhythm and eloquence, the harmony of the verses, prophecies of future events, etc. In other words this Quran has the features of the Quran that was revealed 14 centuries ago and not one person has been able to bring a chapter similar to its chapters. This proves that it has not been distorted or falsified for if it was, it would not have maintained the same qualities.
There is no doubt that if it was possible to bring even one chapter similar to the Quranic chapters the disbelievers that were contemporary with the prophet and were and are known to be the most learned in Arabic literature would have done so, because not only did the disbelievers use every single method to defeat Islam and the Prophet but the Quran itself had challenged them to do so. This way they could defeat the prophet and deny his prophethood so much easier without going through the trouble of going to war with him. Obviously they tried their best but it was useless, for Allah says in the Quran: "Say," Should all humans and jinn rally to bring the like of this Qur'an, they will not bring the like of it, even if they assisted one another."
Another challenge that the Quran has made is that no contradictions can be found in the various verses of the Quran. In this regard Allah says: "Do they not ponder about the Qur'an? Had it been from other than Allah, Surely they would have found therein so many contradictions". There are no contradictions among the Quranic verses we read today.
Also, the current Quran also has all the other qualities that the Quran had 14 centuries ago like prophets' stories, marvelous and valuable truths, prophecies, etc.
This reason is made up of several premises:
1- A revealed religion is a set of beliefs and rules that are revealed to a prophet by God for the guidance of the people.
2- All religions (except for the Seal of Religions), depending on the needs of their time, have partially brought some of the beliefs and rules of the Complete Religion for the people of their time .
3- The reason for having many religions, in addition to bringing new rules from the complete religion, is so that previous distorted ones can be corrected.
4- In order for the last prophet to rise and call the people to the Seal of Religions; firstly, the last religion must be utterly comprehensive leaving out no paths that would lead one to God, and secondly, the last religion must be protected from all forms of distortion. If any of these two elements are lacked in a religion, the mind will not accept it to be the seal of religions.
In this light one can conclude that "the Seal of Religions is immune to distortion", and this means that the book of this religion cannot be altered as well, because the book of a religion consists of most of its rulings and teachings and protecting a religion from distortion without protecting its book makes no sense.
So till here what was proven is that the book of the Seal of Religions will be protected from change, now we will combine this fact with the two other statements (first: The Quran is the Book of Islam, second: Islam is the Seal of Religions) to conclude that the Quran has never been altered or distorted.
Once we accept that Prophet Muhammad was the Seal of Prophets and that his religion was the last religion sent down to humankind until the end of the world we must admit that Allah will always protect Islam and keep it from being changed. How can the distortion of the Quran be consistent with the fact that Islam will stay alive till the end of the world?
Knowing that the Quran meant everything to Muslims throughout time and that they used it as their constitutional law and the rules by which they lived and worshipped Allah, there was no period of history wherein the Quran could have been altered without other noticing.
The Quran was the book that early Muslims would recite in their prayers, mosques, houses and wars when confronting the unbelievers to prove that Islam was the true religion to live by. Islamic history shows that in some cases men would set teaching their brides the Quran as their "Mehr" (dowry).
Generally speaking at that time the only book which was spoken about and so many events revolved around was the Quran. Children would learn it in the early years of their lives and anyone who intended to learn about the new religion was taught the Quran. Therefore there was no chance for the Quran to be changed.
The Quran we have today is the same one that was compiled during the prophet’s time, and even if anyone does not accept this he must know that even those who have different opinions regarding the current order of the chapters and other detailed issues agree that not even one word was added or taken away from the Quran; because Muslims strived to memorize and learn the Quran to the extent that the social status of individuals was somewhat determined by how much Quran they knew by heart.
There were so many Muslims which memorized the Quran that history says that 400 of them died as a result of only one war which took place at the time of Abu Bakr's reign. 
In another war that took place at the time of the prophet and occurred in a village not far from Medina named "Bi’r Ma’unah" history recounts that 70 Muslims who knew the Quran by heart were martyred.
The Quran was not a forgotten book that lied on Muslims’ bookshelves with out anyone reading and reciting it, hence it was not possible for anyone to alter it and it that case it would have been ensued by extreme opposition by the Muslim nation.
Memorizing the Quran has always been seen as a great form of worship and tradition among Muslims. Even after the Quran was compiled, transcribed by hand in the form of a book and later on by print which lead to it being the most sold book in all Muslim countries, memorizing it still meant a lot to Muslims, an inevitable result being that memorizers of the Quran can be found in almost every city and town.
Today there are special schools in Muslim countries with different names like "Madreseto Tahfizel Quranal Karim" or "Jameatol Quran" that have been established for the memorization of the Quran. In some cases the teachers are small children who have just finished memorizing the Quran themselves.
Reports show that approximately one million and five hundred thousand citizens of Pakistan know the Quran by heart! One of the requirements to enter the Islamic University of Al-Azhar in Egypt is to have memorized the entire Quran. After going through a test applicants must score 20 out of 40 to attend classes at Al-Azhar!
In brief, memorizing the Quran is a tradition established by the prophet in his ahadith. Keeping in mind what was said, there is no possibility that the Quran could have undergone any change through the course of 14 centuries.
There were numerous writers which the prophet had assigned to write the divine revelations that were sent down to him by Allah. Historians have mentioned up to 43 Muslims as the writers of the Quran, although Imam Ali and Zeyd Ibn Thabet would accompany the prophet more than others in this task.
The question is how can a book with so many writers be altered?
Since the advent of Islam all of the imams have encouraged Muslims to recite, ponder and act upon the Quran at hand, this comes to show that it was certain it hadn’t undergone any change, especially in the first few centuries of Islam.
In the Nahjul-Balaghah we find proof for such a claim. In sermon 133 we read: "The Book of Allah is among you. It speaks and its tongue does not falter. It is a house whose pillars do not fall down, and a power whose supporters are never routed."
In serman 176 Imam Ali says: "And know that this Qur'an is an adviser who never deceives, a leader who never misleads and a narrator who never speaks a lie. No one will sit beside this Qur'an but that when he rises he will achieve one addition or one diminution-addition in his guidance or elimination in his (spiritual) blindness."!
If the Quran had been changed or altered it would not make sense for the imams to invite the people to the current Quran. If it had been altered it would not have been introduced as a light that helps distinguish between true and false or as Allah's steady and firm rope which we all must hold on to.
Taking these six arguments into consideration, no room should be left for any doubt about the Quran being immune to any form of distortion and falsification.
 See: Payame Quran, Makarem Shirazi.
 See: Al-Mizan, vol. 12, pp 150-155 and 157.
 For further information, see: Mabaniye Kalamiye Ejtehad, Hadavi Tehrani, Mahdi, pp. 54-55.
 See: Translation of Al-Mizan, vol. 12, pp. 150-154.
 Hadavi Tehrani, Mabaniye Kalamiye Ejtehad, pg. 67.
 Tafisir Nemouneh, vol. 11, pg. 22.
 Of course, there are different viewpoints on the compilation of the Quran, see: Question 71, The Compilation of the Quran.
 Muntakhab Kanzul-Ummal as quoted by Al-Bayan fi Tafsiril-Quran, pg. 260.
 Safinatul-Bihar, vol. 1, pg. 57.
 In this regard, the memories and memoirs of Dr. Mohammad Hosein Tabatabai who is famous for being the child who memorized the entire Quran and earned a PhD are worth reading.
 Tafsir Nemouneh, vol. 11, pg.24. Of course, these statistics belong to years back and naturally, the number of those who have memorized the Quran is higher than this number in that country.
 Ibid, as quoted by the Encyclopedia of Farid Wujdi.
 Historians have recorded between 14 to 43 individuals.
 Zanjani, Abu Abdillah, Tarikhul-Quran, pg.24.
 Meaning that when one observes the sayings of the imams, they are all encouraging the Muslims to recite and ponder the verses of the Quran at hand.
 وَ کِتَابُ اللَّهِ بَیْنَ أَظْهُرِکُمْ نَاطِقٌ لَا یَعْیَى لِسَانُهُ وَ بَیْتٌ لَا تُهْدَمُ أَرْکَانُهُ وَ عِزٌّ لَا تُهْزَمُ أَعْوَانُه.
 اعْلَمُوا أَنَّ هَذَا الْقُرْآنَ هُوَ النَّاصِحُ الَّذِی لَا یَغُشُّ وَ الْهَادِی الَّذِی لَا یُضِلُّ .
 وَ مَا جَالَسَ هَذَا الْقُرْآنَ أَحَدٌ إِلَّا قَامَ عَنْهُ بِزِیَادَةٍ أَوْ نُقْصَانٍ زِیَادَةٍ فِی هُدًى وَ نُقْصَانٍ مِنْ عَمًى.
 See: Tafsir Nemouneh, vol. 11, pp. 24-26.