I found two sources sort of answering this question, but conclusions I made of them contradict each other. 
<https://www.reddit.com/r/askscience/comments/29jrvq/what_is_the_smallest_possible_distance_from_pluto/> - here is stated that Pluto and Neptune never come closer than two billions kilometres from each other.

Yet from picture in here (I am aware it might be not to scale) [Can Pluto and Neptune collide anytime in future?](https://astronomy.stackexchange.com/questions/11782/can-pluto-and-neptune-collide-anytime-in-future) it seems that Pluto and Neptune got a lot closer to each other shortly after 1999 than a half of Neptune orbit radius, judging by their speeds and positions in 1999.

So, how is it? How close can they get to each other and when is it going to happen next time?

Tried to use wikipedia, but while article about Pluto contains information at least about when it was in Aphelion, article about Neptune does not any such information (or I could not find it).