We present the first measurement of the energy spectrum of cosmic rays down to 100 PeV from the Pierre Auger Observatory using the part of the surface-detector array with 750 m spacing. This analysis is made possible by a set of additional particle triggers, installed in 2013, which lowers the threshold above which the array is fully efficient by half an order of magnitude. The measurement thus covers a critical energy range around 10$^{17}$ eV where previous observations have shown a change in the spectral index, the so-called second or iron knee. The work relies on a geometrical exposure, a nearly 100% duty cycle, and an almost model-independent analysis, with the energy estimation provided by the fluorescence detector. We discuss the spectrum and the associated uncertainties and compare the results to other studies in the same energy range.