People always overthink this one...
Picking a knife for a soldier is pretty much the same as picking a knife for anyone else.
Guy in an office doesn't have a whole bunch of space in his dress pants for tons of knives; soldier is loaded down with crap and doesn't have much space for tons of knives. For most office or retail workers, and soldiers, a regular EDC knife is going to work for their cutting tasks and not take up too much space.
Now I am not a soldier (never have been, never claimed to be), but I can say for certain I'd be carrying way more knives than most of the other soldiers if I were...because I am a total knife fanatic.

I have two knives with me while lifting weights at the gym.
I carry the huge knives in the woods that others say are too big and heavy, where no one would carry them...and that's in addition to the 2 or three knives in my pockets...and the loaded backpack...and the many liters of water I'm carrying...and the food...
My point is that barring unit restrictions on what can be carried, knowing what that individual soldier is like will be a better guide than any thread about "What knife should I get for a soldier?"
If you do go with a "What knife should I get for a soldier?" thread, then the safest generic answer to be given is to get a regular EDC knife or multi-tool.