I fried the chicken breast in the oil till its surface was brown and then I pressure cooked it. The result was hard chicken. I found it quite difficult to chew.
What should I do next time to make it utterly soft?
Frying the chicken tends to set the muscle fibers by drawing moisture out. An example of this would be taking a thin (1cm) cutlet and frying it until both sides start to turn golden. It will be pretty dry and hard.
Pressure cooking it on the other hand tends to soften the muscle fibers without drawing moisture in (if anything, its putting moisture in).
So, ideally, you should boil/pressure cook your chicken prior to frying in oil (though do make sure to drain the chicken first to prevent splatters!). Additionally, you should undercook the chicken slightly in the pressure cooker since you'll be finishing it in the fryer.
If you are broasting the chicken, then you are going about the recipe incorrectly by frying then pressure cooking it. When broasting, you should pressure fry - if that makes sense.
Breast is lean muscle. Add a ton of heat to it and it'll seize up into a dense brick that nobody wants to eat. In that vein, I'm not sure I'd put chicken breast anywhere near a pressure cooker because that's a really great way to get something to 120°C.
That's about twice what you want. Ideally we want chicken to hit 63.5°C and stay there for a bit. Here's how I'd suggest cooking the perfect chicken breast.
Yeah, that does mean your turnaround time can be 7 hours... But the brining can be done ahead of time (freeze them vacuum-packed in servings) and it delivers restaurant quality chicken every time. It's worth it, even if you have to build your own sous vide (it's quite easy and cheap — that's my tutorial on my blog).
And while I'd suggest doing all three steps, you can pick and choose as it suits you. Slow over-roast, brined chicken is still delicious, just as sous-viding without the brining or hellfire (for salads, etc).