Some old foreign tractors have down pressure. The brand new JD 3 & 4 series have an optional kit for 400ish lbs of down pressure or so. But its very uncommon.
You can get down pressure kits for a 3pt PHD for some of the PHDs. Its a single acting cylinder that's plumbed in a little funky compared to more common hydraulics.
Your easiest solution though is to strap or weld several hundred lbs of weight to the end of the PHD. I hang a 5 gallon bucket full of scrap metal & cement with a chain hanging out to attach to the PHD hook.