The PK-AS is described as:
A well-made Russian sight, good for CQB.
You can pick this up in eastern Itacua province, inside Unidad FOB Armadillo. The “Armadillo’s Commander” mission is nearby. The accessory case is inside the armoury at the front of the base, in the south-west corner.
If you haven’t done the story mission here, or you just want to sneak in and collect the accessory, I recommend doing so at night. All the base personnel will be in the mess tent, except the snipers in the towers which you can take out from a distance first. Then, sneak in and make your way to the armoury around the perimeter to avoid being seen.
I know it shouldn’t matter, but this it the ugliest of all the sights in my opinion, it sits very high on the rail. However, it doesn’t obscure much of your view and the reticle is very clear. It just feels a bit weird to use to me.
I can’t find the zoom for this sight documented anywhere, but in my experimentation it seems to be 3.0x. Try it if you want accuracy for longer engagements, but also perhaps try the Digital Scope (Monte Puncu) for versatility.