Mount Keyboard Tray To Linnmon (Particle Board) desktop?
The screws may or may not hold in pressboard.  It depends a lot on the density of the pressboard and the screws.  There are screws that are designed for pressboard, (Typically a more course thread, but they certainly don't guarantee success.  I think noybman had a good comment that you should test the material in an inconspicuous spot.  

The problem with epoxying wood to the pressboard is that the outer layer of the pressboard is usually some type of laminate that is glued to the pressboard and is designed to provide the aesthetic appearance only and not designed to hold any weight.  To put it another way, imagine placing a piece of packing tape onto a piece of wood and then epoxying a weight to the tape - you are relying on the tape.

You may be able to use a number of screws to attach a piece of plywood to the bottom of the desk.  I would suggest using a piece of plywood that is 3-4 times larger than the mounting surface of the keyboard slide to distribute the weight.  You could reinforce it with epoxy but that would be just extra reinforcement, the screws should be doing the work.

The best way to support the tray is to use bugle head bolts drilled through the desk top.  They would need to be countersunk to be flush on the surface of the desk.  This may be the strongest way, but it is the ugliest and I probably wouldn't do it if it were my desk.