The Earth is 150 million kilometers from the Sun.

Pluto is 6 billion kilometers away, that is, 40 times farther away.

Therefore, the Sun would be seen as 40 times smaller in diameter and 1600 times smaller in area.

As the apparent luminous intensity of a source is proportional to its area (keeping constant light power), it will be 1600 times less bright.

This means that the luminous intensity of sunlight on Pluto’s surface will be 1600 times smaller.

With, on Earth, it is 1360W/m², on Pluto it will be 0.85 W/m², very weak.

Still, bigger than any star.

This corresponds to an apparent magnitude of -18.75.

The brightest star in the sky, Sirius, has an apparent magnitude of -1.46.

The maximum of Venus is -5 and that of Jupiter is -8.

So the Sun as seen from Pluto is still brighter than Venus as seen from Earth.