Linear : adjective [usually ADJECTIVE noun] A linear process or development is
one in which something changes or progresses straight from one stage to another, and has a starting point and an ending point.
There are demishing returns to PRR. This
graph from the wiki shows it pretty well. For more info check out their full section.
PRR.
Long story short - I have played with 200 prr, 250 prr, 300 prr, 350 prr, 400 prr, 450 prr, and 500 prr. At no point do you stop taking damage.
If it were a linear progression then you would eventually hit a "0 damage taken" point. Instead it is a formula that creates a "reduction curve".
As noted in the PRR discussion on the wiki - at some point the reduction provided by increasing your PRR further is outweighed by raw HP increases. This is what it means to have dimishing returns.