You must use an electrical box unless the lighting is low voltage. Very likely, there will be some sort of support (i.e. a small "stud") available to you at some point along the soffit beneath the drywall. If you get lucky when you cut the holes in the plywood and drywall and there are studs directly behind where you wish to install the pendant lights, then you can screw an electrical box directly into this support. As far as options, they make "pancake" boxes that are very thin for lighting applications where there is minimal depth of ceiling material - you just have to make sure you do not exceed the box's wire fill capacity. However, with 3/4" of plywood and likely at least 1/2" of drywall, you are minimally looking at a 1 1/4" deep ceiling junction box (most likely listed under "new work" at the store where there are screened through the back of the box for securing to the stud) and wire fill for your pendant lighting fixtures is probably okay.
Otherwise, if a stud is not accessible once you cut the holes for where you want the pendant lights, you should use an old work box that has an extendable brace attached (example) or is otherwise UL listed for ceiling fixture support. A lot of old work boxes without braces are not listed for ceiling fixture support, so please verify this before installing and potentially later having your lights fall down- most state this inside the box or in the packaging as it is a selling point of the old work box if it is listed for ceiling fixture support. Again, many manufacturers make boxes of different depths to account for situations such as this. The example I linked has different depths of boxes available.
Another option is to use a short depth "pancake" ceiling box mounted directly to the plywood (assuming the 3/4" plywood is fastened securely to the soffit) and then surround it with a circular piece of wood trim that has the same depth as the ceiling box (e.g. buy a round piece of trim and cut a hole in the middle of it for the junction box). Or, a similar option is to get another sheet of trim plywood (probably only needs to be 1/4" or 1/2" plywood max) and cut its dimensions to match the same dimensions of the existing 3/4" plywood and try to stain it to match the cabinets. Then you can cuts holes in the new sheet of plywood where you want your pancake junction boxes to go and secure the new sheet of plywood to the ceiling. This will allow the pancake junction boxes to be fastened to the 3/4" plywood and still have the lights sit flush with the ceiling.
