You should read
this regarding staking.
The key sentence is:
Older and larger sets of coins have a greater probability of signing the next block.
Therefore, if someone has more coins, he has a higher probability to earn stake reward.
What will happen to the PAC in Cryptopia? They will not let us withdraw our coins.
Cryptopia will exchange old for new $PAC coins.
Is there any link of the official statement of cryptopia about the PAC exchange that you said?