This page of the Resident Evil Village guide contains a list of all files to be found in the Stronghold located in the northeast corner of the world map. The following list will help you explore the Stronghold in 100%.
The diary lies on the ground and you can find it after you go through the tower, reach the twisting staircase and squeeze between the walls. You'll find the collectable shortly before reaching the location of the Urias boss fight.
You can obtain the notes by reaching the mini-loch on the way back from the stronghold to the village. Before you use the ladder and leave the dungeon, explore the side location shown in picture 1. The files lie on one of the tables.
You can find the diary inside Otto's Mill adjacent to the stronghold. In order to reach the room with the collectable, open the door secured with 3 padlocks and defeat the ogre. The diary lies on a desk.