Since the basis for the Quran originates from the Torah, in particular Deut 18:18-20:
18 I will raise them up a Prophet from among their brethren, like unto thee, and will put my words in his mouth; and he shall speak unto them all that I shall command him.
19 And it shall come to pass, that whosoever will not hearken unto my words which he shall speak in my name, I will require it of him.
20 But the prophet, which shall presume to speak a word in my name, which I have not commanded him to speak, or that shall speak in the name of other gods, even that prophet shall die.
it would lose its validity since the authority upon which it originated was untrue.
However, if I understand correctly, the Prophet Mohammad could neither read nor write and in such received the writings from an Angel who gave him the written covenant, hence the passage of Isaiah 29:11-12:
11 And the vision of all is become unto you as the words of a book that is sealed, which men deliver to one that is learned, saying, Read this, I pray thee: and he saith, I cannot; for it is sealed:
12 And the book is delivered to him that is not learned, saying, Read this, I pray thee: and he saith, I am not learned.
so therefore, just as with Moses who being learned in all knowledge and wisdom of Egypt, how did he learn how to write the written spoken word since there is no evidence that Egyptians had the ability or used the written spoken word. Instead, the use of hieroglyphics, or the use of symbolic images would indicate that they didn't have the ability to communicate using the written spoken word, in addition they lacked the essential element necessary to begin a written language using the written word.
However, as many that refute the legitimacy of the Quran often cited that Prophet Muhammad did not receive the Quran from the LORD, but from an angel of the LORD, therefore was not a Prophet. Yet there are some that hold that Prophet Muhummad and the Quran represent that third tabernacle spoken of in Matthew 17:4, and Luke 9:33 and Mark 9:5;
4 Then answered Peter, and said unto Jesus, Lord, it is good for us to be here: if thou wilt, let us make here three tabernacles; one for thee, and one for Moses, and one for Elias.
representing the three Testaments, the Torah, the Gospel and the Quran.