CPR will not save anyone from dying on its own. What CPR does is basically manually force blood through the body. It's not as good as a real heartbeat, so you'll still die from lack of oxygen getting to organs eventually. What it DOES do is it serves to slow down the process of dying until real help arrives that can hopefully address the real problems.

In terms of your question, it depends. If the pacemaker is still firing but out of rhythm (called an arrhythmia) and you can keep somebody alive until a defibrillator arrives and it arrives pretty quickly, then yes, they have a decent chance. If the rhythm has fully stopped, no defibrillator in the world will save them, unfortunately.