Travel from CDG to Bergerac

You could either fly Paris-Bordeaux or take the TGV Paris-Bordeaux. Since you're already in the airport, transferring to a Bordeaux flight would be easier than if you were starting from somewhere in Paris itself. Before booking you'd want to compare flying vs. train prices as well as frequency of service.

If the train turns out to be a better value, [I see from later comment that at least some trains leave directly from CDG; otherwise] the TGV leaves out of Gare Montparnasse, which is around the other side of the city from CDG. If it were me, I'd use a car service like Paris Webservices to make that transfer from CDG to the station. You'll want to factor in the cost of that transfer, and allow plenty of time to get there considering city traffic and the possibility of your incoming flight being late. If you fly and book the journey as one ticket with a transfer, then you should probably be protected in case of an airline delay that would make you miss your connection.

From Bordeaux there is frequent train service to Bergerac. Or, if you want to have a car at your disposal during your stay, pick up a rental car in Bordeaux.

If you end up needing to get to Montparnasse station, LeBusDirect Line 4 goes directly there. Or a regular taxi from the taxi stand costs a flat rate of 55 euros for up to 4 people.