There is no investigation going on in this documentary. I've seen just about every Zodiac doc and I'm guessing all the participants in those films think this thing is a farce.
The main problem is the director seems to have done no research, himself. Instead, he features this English professor, Thomas Horan, who is a Zodiac hobbyist. Horan jumps to conclusions and makes nutzo claims like he's the only Zodiac researcher 'who has read all material on the case'. He has no way of knowing that and the claim itself blows his credibility.
At one of the crime scenes, based on the location of a shell casing, he proclaims it proves that the shooter was on the passenger side shooting over the hood of the car. That itself is a huge leap. And from this he 'deduces' there were two killers. This is a ridiculous reach and this guy has NO forensic training, whatsoever.
Horan is not qualified as an investigator and is working backwards from his thesis. That is bad investigative technique and bad science. This is every bit as weak as any of NIck Broomfield's docs on the Biggie and Trupac Murders. If you are going to put out a true crime documentary you have to at least do INVESTIGATIVE JOURNALISM. There is nothing like that going on, here. It's just some retired professor spinning theories without the rigor to drop test them.
The only interesting segment is the AI evaluation of the handwriting but even that is inconclusive. If you watch this 'documentary' you will know less about the Zodiac than if never watched it at all.