I believe it's real (or a translation of a real document) but has been subject to one of the biggest and most expensive cover-up/dis-info operations in history.

It was declared a forgery which means there was an original document somewhere

Eustace Mullins

The attack on the Protocols in the 1920’s proved above all else the truth of their contention… Probably so much money and energy were never before in history expended on the effort to suppress a single document.”

Douglas Reed, The Controversy of Zion

From my own research into the matter, Kerry Bolton has written the most concise/complete summery of where it came from the efforts used to discredit it:

http://mailstar.net/Bolton-Prot-Context-Ed.pdf

Jewish writers such as Israel Zangwill, Herman Bernstein and Norman Cohn insist that the Protocols is a forgery, plagiarised from a satire against Napoleon III by French polemicist Maurice Joly, at the behest of the Czarist secret police in France.

The contrary view is that the Protocols was retrieved from a Masonic archive in France, and that Joly's book was adapted from an earlier version of similar conspiratorial literature in such circles.

...the documents were taken from a Lodge of the Masonic Rite of Mizraim

The entire document is well worth reading.

(It's an abridgement of a longer book that seems to have had a very limited physical run. I've tried to find a digital copy but there doesn't seem to be one.)