For those who doubt that the entire Qur’an is the word of Allah and free from any form of corruption; that it is not the work of any other being, evil mind or whatever, Allah poses a few but vital challenges. These are very brief, because there is no compulsion in believing. Any who believes is for his own benefit while disbelief do Allah and His Messenger no harm. The challenges are:
a) Or they may say, “He forged it,” Say, “Bring ye then ten Surahs forged, like unto it, and call (to your aid) whomsoever ye can, other than Allah if ye speak the truth! Qur’an 11:13
If that is too difficult, then…
b) And if ye are in doubt as to what We have revealed from time to time to Our Servant, then produce a Surah (i.e. chapter) like thereunto; and call your witnesses or helpers (if there are any) besides Allah if your (doubts) are true. Qur’an 2:23

The likeness referred to is in terms of structure, grammar, poetry and so on. Not to talk of the “mathematical miracle” of the Qur’an which may not have been known during the life time of the Prophet Muhammad (S.A.W). Today, it has being discovered that numerically, the Qur’an revolves around the number nineteen (19) which has its origin from: “Over it are nineteen.” Qur’an 74:30
For example:
a) The opening verse of the Qur’an (popularly called Basmala) has 19 letters.
b) The Qur’an has a total of 114 chapters i.e. 19 x 6.
c) The first chapter to be revealed (Surah 96) is the 19th from the end; and has 19 verses.
d) The first verses of the Qur’an to be revealed are the first five verses of Surah 96 and the total number of words in these verses is 19.
e) The final chapter to be revealed (Surah 110) consists of 19 words.
f) The word Rahman (All-Merciful) appears 57 (19 x 3) times in the Qur’an.
g) Etc. etc. etc.

For more on this, here are some links:

Therefore, anyone who is in doubt that the Qur’an is a revelation from Allah is urged to contemplate his insistence, if it is wise to treat lightly by dismissing what cannot be rationally or intellectually rejected. “Is it such a Message that ye would hold in light esteem? Qur’an 56:81