Copper Mill - Collectibles Near the Large Room with One Open Wall

Loot all the bodies and pick up various supplies around this room. Jump across the gap in the floor, search the bodies on the other side, grab some rifle ammo and salvage, and then scramble up to where the sniper was standing.

Off to the right is a basket of herbs and a table with a document (23/26), An Ally in the Darkness from the Red Arrival series (Russian).

Now grab a bottle and head up to the next level either via the inside staircase or by climbing up from the exterior ledge. At the top of the next yellow ladder...

...is another document (24/26), Intercepted from the Conspiracy Theories series, sitting on some wooden boxes directly ahead.

Down the stairs to the right is a flammable barrier. Use the bottle to make a molotov cocktail and burn down the barrier.

Or, starting from the ledge below the ladder, jump toward the wall and scramble onto the ledge above.

However you get there, you'll find a salvage crate; a cosmonaut poster (5/7) you can burn for the challenge using another molotov; and a relic (18/18), a Pillbox from the Red Army Officer set that gives you both XP and Russian proficiency.

Burn down the barrier if you didn't do so before. Pick up another bottle and return to the level above.

At the top of the stairs, turn around to spot another cosmonaut poster (6/7) high on the wall. Make another molotov and use it to destroy the poster.