Over the course of the story you can find several inventory upgrades. They are big camping bags and an easy location to find one is in the old house. There is a single save point in the old house, which you can see on the map, the bag is located directly next to the save point. Picking it up will unlock your achievement / trophy and give you 4 more inventory slots.
Here is a video guide with English commentary!
Here is a playlist of guides for this game!
https://www.youtube.com/playlist?list=PL-HeW6XkKdUKUtj7CV_kW...