By means of path integral Monte Carlo simulations based on a coarse-grained model parameterized by first-principles calculations, we reproduce the large isotope effect observed in H-bonded KH2PO4 (KDP) and deuterated DKDP, showing that the dramatic shift in the ferroelectric critical temperature T-c upon deuteration is governed by the effective mass and local geometry of hydrogen bonds. We find clear evidence of a quantum paraelectric phase in KDP-type ferroelectrics at high pressures, characterized by a universal critical proton off-centering parameter (delta(c)), independent of isotopic substitution. Remarkably, this universality explains the experimentally observed collapse of the isotope effect under pressure, reinforcing the essential role of geometrical effects. Our results suggest that the emergence of quantum paraelectricity and the universal linear behavior of T-c with delta across chemically diverse systems share a common microscopic origin associated with local proton delocalization within the hydrogen bond network.