We made a series of isotropic pressure-densified (0-200 MPa) amorphous homopolymer [atactic polystyrene (a-PS)] and copolymer [poly(styrene-co-acrylonitrile) (SAN) and poly(styreneco-maleic anhydride) (SMA)] glasses and studied their macroscopic pressure-volume-temperature (PVT) properties vs their free-volume characteristics from the Simha-Somcynsky equation-of-state (EOS) theory and from positron annihilation lifetime spectroscopy (PALS). The glass densities lie in the range of 1.0403-1.0535 g/cm(3) (PS), 1.0573-1.0759 g/cm(3) (SAN), and 1.0989-1.1196 g/cm(3) (SMA). With increasing formation pressure, all pressure-densified glasses exhibit decreasing volume and free-volume characteristics such that the changes in specific volume (1.26%-1.85%) are < ortho-positronium (o-Ps) lifetime tau(3) (5.5%-9.1%) < free-volume hole size V(tau(3)) (10.3%-17.1%) < free-volume fraction h (25.1%-30.5%). We find, furthermore, that the o-Ps formation probability I-3 is independent of formation pressure. Likewise, the glasses' thermal expansivity alpha(0) remains constant. There is a one-to-one correlation of tau(3) and of I-3 when calculated from POSITRONFIT and the maximum entropy lifetime (MELT) program. The full width at half maximum of free-volume distributions determined from PALS data and MELT does not change with formation pressure within established limits of uncertainty. Neither h vs V(tau(3)) nor h vs bulk modulus K, calculated from the Tait EOS of the glasses, show unique relations that are common to all the studied glasses. On the other hand, K vs V(tau(3)) gives a universal curve for all pressure-densified glasses from this study and from our previous study on PMMA: K=8.190-4.479x10(-2)V(tau(3)) (r(2)=0.92). Moreover, three more polymers from the literature are well described by this curve. The bulk moduli increase by up to 11.5% (PMMA), 7.6% (PS), 11.2% (SAN), and 10.2% (SMA) and they follow the order PS < SMA < SAN < PMMA.