What to do to get tender, edible steak instead of it being rubbery? I've seen this, but it does not answer my question.  

This is with regard to the Kerala beef fry or even Kerala beef curry; not the western way of cooking steak.  

The steak pieces: 1kg bought from a local beef shop where they just take a random piece of meat, chop it into little pieces of around 2 or 3 cm in size and give it to you.
What I did: Kept it in freezer for two days, took it out today and put it in water and heated the water a little. Pulled apart frozen pieces slowly in a span of an hour and allowed it to reach room temperature.
Recipe: Added spices as per this recipe and left for marination for an hour.
Pressure cooker: Put beef in cooker and heated on slow fire for 35min until there were 4 whistles/steam-let-outs that happened in a span of 2 minutes. Allowed pressure to go down and after around 20 min, opened the cooker to find the meat very rubbery. Appeared slightly cooked on the outer layer for 1 or 2 mm, but the meat inside seemed raw and when I try to tear it, it feels tough and rubbery and extends a little like when you start pulling chewing gum apart with both hands.  

Beef meat ends up like this every single time I try to cook it in a pressure cooker. I'm sure I didn't overheat it and it certainly wasn't cooked for too long. Or is it wrong to cook it in a pressure cooker and instead boil it in an open top container? I hadn't added vinegar or wine or lemon juice during marination. Could they have made a difference?  

ps: I continued heating the beef with its gravy for another 20 min on low flame, but it didn't seem to make the meat any better. So I separated the gravy and roasted it as in this recipe. The taste turned out excellent but the meat was still rubbery. So it would be much appreciated if you could help with info on ensuring that the meat is tender and well cooked when we start cooking it itself.
Starting with a "random piece of meat" may be part of the problem. Some cuts are more suitable for this than others.

If the meat seems "raw", then something is very wrong here. An hour at pressure-cooker temperatures is more than enough to over-cook it. There's no way it could be raw.

I suspect that over-cooking is the problem, and that will depend on the cut you're using. I suspect you've got a piece with little fat or connective tissue, and that's not going to soften in a pressure cooker. It shouldn't seem "raw", though; it should seem over-cooked.

In the future, I'd suggest looking for a chuck roast or other fatty piece. If you do end up with a cut like bottom round, it may help to cut it into thinner pieces, across the grain. That mechanically tenderizes the meat by snipping the chewy fibers.

Either that, or something is desperately, weirdly wrong with your pressure cooker.