You don't if you have a credit card (a pre-paid one might work, too). Just place your linden order, select 'pay with paypal', and the paypal interface should give you an option to enter a payment method. Just enter the credit card here.
Do not link your bank account to paypal, ever. It will let you keep making transactions after you're overdrawn without telling you. I once racked up 500 dollars in non-sufficient fund fees. The bank thankfully reversed it.