You have to know your equipment. You should be able to answer your own question about engine braking.
Gas engines have engine braking. For diesels, I know there are engine ("Jake") brakes, exhaust brakes, and transmission retarders to help the service brakes slow the vehicles down. Either you have these systems or you don't. I've only driven with Jake brakes, so I can't speak for the others. It's been a while, but as I recall there was practically no slowing with the switch in the "off" position, and I think downshifting would only overspeed the engine.
If you have one of these, the safety guidance is to go downhill in the same gear (and approximate speed) you would need to use to drive back up.
Also, make sure your service brakes are adjusted, especially air brakes. If it takes all the travel to make the shoes expand to reach the inside of the drums, you'll still stop fine in normal driving. Metal expands when heated. I've read that the direction of expansion of brake drums is away from the shoes, so going down a long hill it's possible for badly adjusted brakes to lose the ability to put pressure on the inside of the drum. You can't just make another pump on the treadle like you could to pump more fluid to badly adjusted hydraulic brakes.
With any type brakes, don't just hold your foot on the treadle going down the hill. This will surely overheat the brakes and diminish braking ability. I lost braking in a compact station wagon once after coming down a mountain pass. It wasn't fun to be unable to stop at a stop sign in the city, no matter how hard I pushed the pedal. It was a good thing no one was coming. (What doesn't kill you makes you stronger, or at least smarter.)
Learn about "stab braking." Brake hard down to 10 MPH/16 kPH below your target speed, say 40 MPH, then use engine braking only until your speed reached 50 MPH, and brake again. This allows some brake cooling during the pauses.
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.