Better than any other scripture: manuscripts dated to the Prophet's ﷺ own era match today's text (Qur'an 15:9), and the mass memorization never broke. By contrast, textual critics count hundreds of thousands of variants across the Bible's manuscripts.
Mainstream textual scholarship accepts the Uthmanic origin of today's text; the history of the earliest companion codices (e.g., the Sanaa lower text) is still actively debated among specialists.
Didn't He Say