What reason do we have to believe that this is real? Does anyone know?
Major news outlets have it in their possession, have published a variety of select quotes that are found in this posting, have described the length of the manifesto as matching this posting, and have not denied the this published version was the manifesto found on the suspect. If that's not enough, then give it a day for more information to come out.
Their source/verification of validity is "trust me bro".