It depends what kind of atmosphere. Pluto is cold enough (even at it's hottest) that CO2, NH3 and CH4 are mostly solids. (look up boiling temperature of each and compare to Pluto's temperature if you like). There's some photo-evaporation, and perhaps some equilibrium for the heavier gases, but Pluto's primary atmosphere is Nitrogen, which has a lower boiling/freezing temperature. (the atmosphere is too thin for it to exist as a liquid).
Based on this chart and Jeans escape, Pluto would need an escape velocity of about 2.5 km/s (roughly the mass of Io, or about 7 times it's current mass) to retain it's lighter gases like CH4 and NH3 as well as it's nitrogen. (Nitrogen would require a slightly lower escape velocity). Very roughly speaking, escape velocity can be tied to mass. If it was closer to the sun and hotter, it would need to be more massive.
This is, of-course, a ballpark answer.
With no magnetic field, pluto would still lose some atmosphere to the solar wind, but that far from the sun, that loss would be slow. It wouldn't lose much atmosphere to heat-escape (or Jeans-escape), like it does now. That's not to say Pluto would have a significant atmosphere if it had 7 or 8 times it's current mass but it would likely have a fair bit more. Pluto's problem is that it's cold enough that much of it's atmosphere would freeze. If it could get a thick enough atmosphere to maintain a permanent greenhouse gas effect, then it might maintain a real atmosphere. Again the minimum mass required for that would be in the 7-8 times it's current mass range, but the atmosphere would also need to be sustained and not freeze on it's surface. Significantly more mass and a higher escape velocity would make retaining an atmosphere much more likely.
The colder the outer edge of a planet's atmosphere is, the lighter the planet (or moon or dwarf-planet) can be and still retain an atmosphere but there's also the problem of too cold and the atmosphere will freeze into ice on the surface. Titan is a good example of this. It's far enough away from the sun that it can maintain an atmosphere, mostly formed by out-gassing from it's icy surface. A few billion years ago Jupiter's 4 Galilean moons may have had atmospheres similar to Titan, lost over time due to heat from the sun and perhaps also from Jupiter's very active radiation belts.