The planet Earth has a slightly eccentric orbit. This means that its distance from the Sun can vary slightly as the Earth travels an elliptical path around the Sun. Pluto has an extremely elliptical orbit, varying its distance dramatically from the Sun dramatically.
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So the closest distance between the Earth and Pluto occurs when Earth is at its most distant from the Sun, and Pluto is at its closest. And the Sun, Earth and Pluto are lined up in a perfect line. When this happens, Pluto and Earth would be separated by 4.2 billion km.
At their most distant, Earth would be at its furthest at the opposite side of the Sun from Pluto. At this point, Earth and Pluto would be separated by 7.5 billion km.
And so, the distance from Earth to Pluto ranges between these two distances.