Soft brake pedal is usually caused by air trapped in the lines. That suggests that you have a leak somewhere. The air got in when fluid ran out somewhere. You need first to check for leaks at all four wheels and you need to look at all the lines, too. Probably best to put it up on a lift for this. The cure is to fix the leak, then bleed the brakes. If you just bleed the brakes, you won't be fixing the problem, just the symptom.
If there is no evidence anywhere of a leak, and the fluid level in the reservoir is steady, then look to the master cylinder. If the pedal goes farther down one time, and then just a little the next, it could be a cup starting to leak in the master cylinder. If the calipers, wheel cylinders, lines and hoses check out OK, then replace the master cylinder, and bleed the brakes.