how to withdraw the bitcoin from the wallet created programmatically

[flyingbird2017]

I need to create web wallets in my web project for each user. The scenario is as below:

1.The user creates an account.
2.The project automatically Create a wallet for each user.
3.The project automatically send some bitcoins to the user's wallet.

The questions are:
1.Is it an appropriate to use bitcoinjs to create the wallet for each user?
2.How can the user back up the bitcoin wallet?
3.how can the user take out the bitcoins from the wallet?