It is hidden in the safe in Ramos' house, in the room upstairs from which your target will escape (during side mission).
This secret is located in the chest text to the other treasures (during side mission), in a cave next to the fight with the mini-boss.
Near the store (500 meters) you will find a destroyed wagon and a chest text to it. The secret is inside.
Near to the building where Ramos will hide (at the end of that mission). Look around from the place, where the building is located. You should see rocks about 300 meters from you. Behind them is a small cemetery and in one open coffin you will find the secret (during side mission).
Being next to the store with guns look around. You should see burned building. Move close toward it and you will see the chest.
The next secret is also nearby the store. This time go behind it and try to find a tree (which won't by hard). Next to it will be sleeping Mexican man with the sought after item. Steal the scroll and kill him.
Nearby the store, going down the hill you will encounter a water tower (in the background of the picture). Being there look around and try to spot gallows. Next to it is the scroll.
During the task with stolen cattle you will reach a narrow path which leads you toward enemies, next to the edge. Being in the beginning part of that path try to find a dead tree. Lean out there to see what is below. You should notice a ledge with the scroll (during the task).
After finishing the task with stolen cattle, climb the hill at the front of the camp (behind the field with cows). You will found yourself on the top of the hill where this two pictures were taken. The left screen is showing the place with the fight whereas the right one the other side of the hill with a small house in the background. You will find the scroll right there.
After leaving the hideout of bandits who stolen treasures turn right and go along the line of rocks. In that way you will reach scaffoldings. Playing as Thomas climb the right side of it with the lasso. Playing as Ray climb left side (two ladders) and covering a ledge, blow up a rock. The secret is hidden in a bucket.
After leaving the hideout of bandits who stolen treasures turn left and go along the line of rock. In that way you will reach a closed mine. Next to the buried tunnel is a tram and inside it you scroll.
Another secret is on the roof of the white house near the store (behind the hill, about 500 meters away).