What's the best way to break large denomination Euros to smaller ones in Alvor?

I have some 100EUR notes that I brought from my bank back home and would like to change them to 10 and 20 EUR notes. Will money exchange places do this for free or is there a bank in Alvor that would do this? I'd hate to give a restaurant or store a larger note when your purchase is much smaller.

Any advice will be much appreciated. Thanks.