PH adjusters work simply by altering the pH of a solution. If the current pH is at 5 (slightly acidic) a solution of alkaline (pH UP) is added to raise the solution to the desired level. This also works in reverse.
Although 20% Phosphoric Acid is quite a bit less concentrated than 85%, it may still be fairly strong when mixing into smaller batches of nutrients. Always test with small amounts at a time checking the PH level each time as well to see how much it was lowered. Add small increments at a time until you reach the desired PH level.