What is the most important document in history?

I assume by "document" you also mean books. However, the first thing we need to do is define "important." Do you mean the most widely read document, the document that has been exported to the most countries, the oldest document that is still relevant, or the document that has sold the most copies?
As you can see, it is a very subjective question, and a person can make the case for many documents, but I will focus on a document's social impact. For example, Quotations from Chairman Mao Tse-tung, also known as "Mao's Little Red Book," was distributed widely in China during the country's Cultural Revolution. Given China's huge population, we can estimate that hundreds of millions of copies have likely been distributed or sold since the 1970s. For Chinese citizens, not accepting the book was tantamount to rejecting the message of Chairman Mao.
The Christian Bible is the biggest-selling book in history, has been translated into the most languages of any book, and has been distributed or sold in every country on earth. But the Bible has also been distributed by missionaries to countless people who did not request it. So, it, too, has had a wide distribution, but not necessarily because people asked for it. However, I would say that, given its impact on every aspect of life in the West and throughout untold countries in the non-Western world, it is arguably the most important document in history. But, again, we are defining "important" in this instance as a document that has had the biggest social impact.