Depending on the rifle, 2-3 lbs is about the minimum safely possible for a military large action - we're not talking the set-trigger versions here.  How involved?  Could be as easy as polishing/thinning the trigger return spring, to quite involved.
You really have to be careful with the sear surfaces, as these will also change block position for loading, if you take material off....  Also, the sear angles are critical, even a slight change could make the rifle unsafe - or make the trigger heavier.  Depending on your mainspring, it could be possible to clip a coil off, although that is not really recommended unless you have a spare spring handy.
Some people will also reduce sear engagement with a set screw in the cocking lever sear stop.
Best bet is to leave it to someone who really knows these rifles, if you're not mechanically inclined and/or don't want to spend the time needed to study the action workings and relations in detail.
Practically, it is hard to get appreciably below 3 lbs - but usually, it is easy to get a nice, clean trigger pull.

Don,

In the Jan.-Feb. 2005 issue of the Journal I started a series on building a rifle on a large frame Martini.(and if time ever permits I'll finish the series) In that first installment I showed how I do a trigger job on one and get a two pound pull. The action started out with a 16 pound trigger pull. I worked on the springs and the sear. If you have any questions after reading the article be sure and ask.

Steven Durren