Using panchromatic data ranging from X-ray/UV to radio, we performed detailed plasma diagnostics and abundance analysesof the planetary nebula (PN) NGC 6781 to assess the physical conditions of each of its ionized, atomic, and molecular gas and dust components. Empirical nebular elemental abundances, compared with theoretical predictions via nucleosynthesis models of asymptotic giant branch (AGB) stars, indicated that the progenitor was a solar-metallicity, 2.25-3.0 Solar mass initial-mass star. We derived the best-fit distance of 0.46 kpc by fitting the stellar luminosity (as a function of the distance and effective temperature of the central star) with the adopted post-AGB evolutionary tracks. Our excitation energy diagram analysis indicated high-excitation temperatures in the photodissociation region (PDR) beyond the ionized part of the nebula, suggesting extra heating by shock interactions between the slow AGB wind and the fast PN wind. Through iterative fitting using the Cloudy code with empirically derived constraints, we found the best-fit dusty photoionization model of the object that would inclusively reproduce all of the adopted panchromatic observational data. The estimated total gas mass (0.4 Solar masses) corresponded to the mass ejected during the last AGB thermal pulse event predicted for a 2.5 Solar mass initial-mass star. A significant fraction of the total mass (about 70%) was found to exist in the PDR, demonstrating the critical importance of the PDR in PNe that are generally recognized as the hallmark of ionized/H+ regions.We would also discuss implications of this exercise in a larger context.