Do I need to link my bank account to my PayPal account before I top up?

Yes, if you're topping up from a bank account other than a DBS/POSB bank account.

If you're topping up from a DBS/POSB account, you won't need to link your bank account to your PayPal account. However, when you top up for the first time, you'll need to set up PayPal as a payee in your DBS/POSB account.