KHOLAT

Dont panic if you dont get it right after Act 2 ends, the achievement unlocks at the end of Act 3, right before the credits.

A good way to tell you will get it is if you got all 3 achievements related to gathering all the other collectibles.

I used a steam guide, it was pretty useful although some notes were on a ledge above me when I got to the spot on the map, so its not 100% accurate, you will need to search around a bit. You will notice a note is nearby because you can hear paper fluttering, if you dont see it right away its probably above you on a ledge so move around and try to circle your location upwards.

If you turn off your flashlight, the note will slightly glow blue or red making it easier to spot.