So I might have messed up. I designed a PDC with 10 terraforming modules on it, produced it, than sent it to mercury and used a construction brigade to actually put it up in something like 5 years (in retrospect simply making ships would have been much faster but I wanted to try something new).
Issue is, it's up there, in orbit. And it does NOTHING. It has fuel on it (even though I don't even remember if that's needed), is properly built up, has the modules. But doesn't actually provide any "terraforming" on mercury :/. Any ideas?
EDIT: Semi-ish fixed. Apparently the terraforming doesn't work if the PDC is in the original TF it spawns in (after you build it), in other words after making it you have to move it from [planetname]:PDC to say "Terraformer" TF. The game was giving me sass about it so I had to use SM and essentially "teleport" the PDC into a different task force (even though it stayed in orbit of the same object) after that it started working.
It's still faster to just send in terraformer ships rather than build a PDC since construction brigades are absurdly slow but I think an orbital terraforming platform that can be refitted into a defence platform after the planet is settled has some serious cool factor to it.
One of the biggest problem with sticking terraforming modules on a PDC is that you can't move them somewhere else when you are done. PDC are permanent installations once constructed so using terraforming modules this way is sort of pointless, at least when you compare of just slapping them onto a station and drag them there with a Tug, or just stick a commercial engine on them so they can go to wherever they need to be by themselves.