We propose to observe the far-infrared cooling lines of CII (158 mu), OI (63 and 145 mu), H2 S(1) (17 mu), S(2) (12 mu), and Si II (35 mu) in Photon-Dominated Regions (PDRs) associated with reflection nebulae exposed to radiation from cool stars with T_eff=5,000-22,000 K. Compared with classical PDRs illuminated by early O and B stars, these clouds close to A and F stars receive much less far-ultraviolet radiation. Since the incident radiation field controls both the heating and the formation and destruction of atomic and molecular species, the observations will allow stringent tests of PDR models in a regime that has so far been unexplored. In particular, the relative contribution of the various types of grains (including polycyclic aromatic hydrocarbons (PAHs)) to the heating can be investigated since the absorption cross section of PAHs has a different wavelength dependence from that of large grains. The dominant cooling lines of PDRs occur in the far-infrared and hence cannot be observed from the ground. In general, the expected line strengths for the reflection nebulae in our sample are 1-2 orders of magnitude less than those observed in these classical PDRs. Hence, the high sensitivity of ISO is essential for this study.