On this spoiler-free page of our guide to The Last of Us 2, you can find a list of all secrets available in The Overlook stage, which is part of the Jackson chapter. Unfortunately, there is only 1 collectible in this mission and you can miss it quite easily.
List of collectibles available in The Overlook level
There is only 1 secret that can be found during The Overlook stage, namely:
It's an artifact. Abby has to get to the point where she needs to crawl under the building and shoot the infected one.
After the crawling sequence, find a shutter to access the interior of a nearby building.
You can find the document on a table in one of the rooms of the cabin.