Thinking about adding a Keyboard tray to a MDF Desk, is it safe?
If you use coarse threaded screws specifically engineered for MDF and use all the mounting holes available to you, the load distribution should be good enough for the desk. If you really want the belt-and-suspenders approach, consider to mount the sliding arms to a pair of one-by-twos or a sheet of plywood, then glue and screw them to the MDF. This will most certainly prevent pull-out of the screws holding the arms, as well as provide for longer screws if you consider that you now have the thickness of the one-by-twos or the plywood added to the thickness of the MDF. If you plan to screw into the MDF, pre-drill and use screws that will not puncture the top. MDF likes to dislodge if the screw tip gets too close to the surface.