no, they cannot withdraw from a linked account without your permission from an initiated transfer. any funds you leave in your temporary paypal holding are theirs for the taking, of course.
If they decide to reverse a payment to your paypal account and you do not have enough funds, it will start to transfer from your bank account tied to the paypal account, without warning or consultation.
There are stories about this all over the web. You essentially agree to it in their TOS.
I need to look into that more. Maybe open a 2nd checking account somewhere else and tie that in.