What is each pin's output voltage of an ATmega328P?
You're on the right page, just didn't go far enough down. \$V_{OH}\$ and \$V_{OL}\$ are the parameters you're interested in. In practice, it will be very close to \$V_{CC}\$. If you're trying to draw some current from the pin, it will drop some. The datahseet uses \$V_{CC}\$ = 5V and I=20mA. In that case, the output voltage on the pin is guaranteed to be greater than 4.2V at 85C. If you're just going to be driving some high impedance pins on another device, you don't really need to worry about it.