A non-monotonic dependence of the thermoelectric properties of PbS epitaxial films grown on (001)KCl substrates and covered with an EuS protective layer on the PbS layer thickness (d = 2-200 nm) was detected at room temperature. The complex character of the dependence on d is attributed to a competition between percolation phenomena and size quantization. Within the framework of percolation theory on the basis of experimental data, the critical exponent for the electrical conductivity was determined. Oscillations in the thickness dependence of the kinetic coefficients are attributed to quantum size effects occurring in thin layers.