Track saw + new or sharpened blade AND a scoring cut (remember to go "backwards" doing this!). I find the resulting cut doesn't even need a pass with a plane.
It helps that my Makita track saw has a scoring setting, whch makes that bit easy, but even so, it's not a big deal.
[edit] You can do the same, door-still-hung- trick (in the video Doug linked-to above) with Makita, with the additional feature that Makita rails let you lock the saw on the rail (anti-tipping feature). I'd guess those Festool clamps would probably fit the Makita rails too. [/edit]
For the final cut I'd suggest having the blade deep-ish, so the teeth enter the door at a decently steep angle - this minimises splintering.
But don't forget: if these are modern interior doors, the middle bit is basically thin cardboard strips on edge in a honeycomb pattern (simply to space the two main surfaces apart), with small and nasty bits of softwood all round. If the doorway is small, the actual door might already have had a lot of the bottom softwood trimmed off in fitting, which is a right PITA to deal with.
I had to trim four doors in my daughter's house recently for carpets, and was annoyed to cut through into the honeycomb on two of them. The builder who fitted them left the tops tall (uncut), making further trimming for carpet clearance almost impossible. What made this worse was that the softwood strips weren't even fitted parallel to the door's original edges, but on a squint. So I ended up with a thin wedge of splintery softwood on the bottom!
In retrospect, I should have drilled through the hinge side with a small jobber drill, to see how much wood was left on the bottom of the door - if it hit free air, obviously, no bottom batten at that point! Such small holes could then be easily filled for painting. If there isn't enough wood left, I guess the only option is to trim the top, move the hinge positions, then trim the lock and hinge sides as necesary (frames are rarely true) - PITA.
The doors came from Wickes, BTW. Abysmal quality (even if every other shed sells the same stuff, that is really no excuse).