It is only optional to determine Kazuhira Miller's whereabouts, but you should not ignore it, because it is a part of the tutorial and, apart from that, you complete one of the secondary objectives, this way. You start the mission in Spugmay Keep, in the Southern part of the map. Follow the instructions and pick Ocelot the binoculars and use it to locate the Da Wialo Kallai village in the distance. At the same time, practice zooming in and out, gaining additional information on the object within sight and marking of points of interest (the only purpose of that is to navigate the environment better).
After Ocelot leaves the area, head to the village. On your path there, you run into one of the outposts (number 23) - you can either walk around such locations or try to secure them. In the case of the latter, check the outpost, from a safe distance, to learn about the positions of the enemy soldiers and attack afterwards. In the outposts, you can find ammo, diamonds or resources.
The best direction, from which to approach the Da Wialo Kallai village is the South or the West. Make it over to a higher vantage point and start your reconnaissance. Apart from finding positions of enemy soldiers and marking them, try to find the central building shown in the above screenshot, which you need to reach.
After you have examined the area, you can step into action. While heading towards the building with the documents, you run into two, or three, enemies and you need to be especially careful with the guard right next to the destination. It is a good idea to eliminate the enemies with the pistol, but you need to get sufficiently close, first. After you reach the building, climb up the stairs and enter the small room, which should be empty now. The documents are on the desk and you need to interact with them, which starts a short cutscene.
Note - If you find it difficult to stay in the shadows, while exploring the village, wait until dusk and then locate the generator in the village, to sabotage it. This will win you enough time to reach the file in complete darkness.