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Assuming your flight to Paris departed from a non-Schengen country, you will pass through Schengen immigration when you land in Paris. After clearing immigration, you will be free to leave the airport if you wish. CDG is a long way from the city center; Orly is closer in.
With your German visa, you can enter the Schengen area through any Schengen country. Entry through France while on your way to Germany is perfectly OK.