Much has been written on this forum about converting US dollars to pesos in PDC. Unfortunately, much of the "information" posted here consists of suppositions, rumors, and second-hand information that has gradually been modified when transmitted from person to person.
My wife is in PDC at the moment, having arrived Saturday morning. Before she left, I asked her to change USD to pesos at a cambio and at a bank, so we could get first-hand information as to rates, required documentation, etc. Her experiences, relayed to me on the phone last night, were almost exactly as we expected them to be.
The cambio DID NOT require her to present her passport. They're not tied to the central computer system as banks are, so they have no way of knowing how much money you have changed before showing up at their window. Banks do, of course, require you to present your passport and they track your currency exchanges to insure that you don't exceed your USD $1,500 per person per month allowance.
Yesterday, the rate at the bank (Scotia Bank on Constituyentes) was 11.4 pesos to the dollar. The rate at the cambios was 11 pesos to the dollar. If you change USD $1,000, you'll get 400 pesos less at a cambio than you will at a bank. That's just a bit over USD $36 at 11 pesos to the dollar. Considering the time and effort required to change currency at a bank, I'll gladly pay the $36 premium to change at cambios.
My wife has an aversion to using her ATM card, so I can't report on the rate/charges when obtaining pesos from an ATM. I'll be joining her in two weeks, and I'll determine and post the ATM rate/charges at that time.
She also hasn't been to Mega yet for the initial grocery shopping trip. Our initial trip to Mega is almost always over USD $100, so that will be a good opportunity for her to confirm if there is indeed a limit on using USD there. I suggested that she take additional pesos, for insurance.