A: Yes. Plutonium uses its own plutonium-bootstrapper-win32.exe and a specific steam_api.dll designed for their servers. Do not mix the vanilla Steam DLL with Plutonium, or vice versa.
If you suspect your antivirus deleted the file: Download Steam Api.dll For Call Of Duty Black Ops 2
A: No. 90% of these files are fake. Always use Steam's verification tool. A: Yes
C:\Program Files (x86)\Steam\steamapps\common\Call of Duty Black Ops II\ Download Steam Api.dll For Call Of Duty Black Ops 2
If you legitimately own Call of Duty: Black Ops 2 on Steam, You already have a legitimate copy waiting for you.