First, you're going to need a very big lens.
Pluto, being much farther from the sun than Earth, gets much less sunlight, and this is the first requirement to address. How much light does Pluto need? Let's start by assuming it needs as much sunlight as Mars, since there is some possibility that Mars could be terraformed.
Mars orbit is about 142 million miles in radius. Pluto's orbit is about 3.67 billion miles in radius, or about 26 times larger. Since light intensity falls off as the square of the distance, Pluto will need a lens about 670 times the diameter of Pluto to concentrate enough sunlight to get the equivalent of Martian light levels.
If that seems unpalatable, and you want to wave your hands a lot (keeping Clarke's Law firmly in mind), you can specify a series of artificial suns (much smaller than the real thing, of course) orbiting Pluto and keeping it warm. The problem with this is that, even though the artificial suns are much smaller than the real thing, they nonetheless put out a LOT of energy. As a rough number, you can start by looking at sunlight levels on earth, a total of about 174,000 terawatts. Pluto is about 1/5 the diameter of Earth, so it has 1/25th the area, and would need 1/25th the power, or about 7,000 TW. However, the artificial suns would lose at least half of their radiated energy to outer space, and actually something like 9/10 is a better guess, so the total power from the suns would need to be on the order of 70,000 TW. This is roughly the equivalent of an 18 MegaTon nuke every second, but with none of the radiation which a nuclear weapon produces. While the proportion varies a bit with bomb size, about 80% of the energy released by a nuke is gamma and soft x-ray radiation, neither of which is good for you. So even if you could filter out the harmful radiation produced by the suns, unless you could convert that harmful radiation to something useful, you'd need something like a 90 MegaTon nuke going off every second.