Here is the full list of all 10 PAC-MAN 256 achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Fully upgrade every powerup
Score a 256 in singleplayer
Complete 16 missions
Unlock every powerup
Fully Upgrade a power-up
Eat 64 Fruit pickups in singleplayer
Earn a x16 Power Pellet multiplier in singleplayer
Eat 64 Ghosts while using a Power Pellet in singleplayer
Eat 2048 Pac-Dots in singleplayer
Play a multiplayer game