The Master Key allows Pac-Man to open any fruit door without having said fruit.

Pac-Man World Re-Pac is based on the Japanese version of the game, so the grand score required to obtain the Master Key is 780,000 instead of 1 million.

Grand Score does not transfer between save files; To ensure you get efficient high-scores across every stage, you should:

1. Get all fruit.
2. Get all letters and beat the bonus stage.
3. Kill as many enemies as possible.
4. Earn at least one 1600 ghost combo in each maze.
5. Complete the stage without dying for a 2000 point bonus.
6. Hit the Jackpot in the slot machine at least twice per stage.

I was able to do it without #5, but it will help.