The saga of HTS has been recently enriched by the detection of the pair density wave (PDW) order. Such emerging order characterized by Coopers pairs with finite momentum has been directly observed for the first time by STM in 2016. In agreement with theory, STM experiments determined the PDW wavelength to be twice that of CDW. However a modulation with a wavevector Q=Q_CDW/2 has not been observed by X-ray scattering experiments so far. A recent RIXS measurement on underdoped YBCO seems to solve this contradiction: by isolating the quasi-elastic contribution of the RIXS spectra along the (H,0) direction from the background, we revealed both a CDW peak at Q_CDW≈0.32 rlu and a second peak at Q≈Q_CDW/2. Here we propose to study with RIXS the latter peak, as function of temperature T and doping p, in YBCO thin films. The T and p evolution of the peak will tell us if this peak is the long-searched PDW peak, possibly giving hints on its connection with the CDW and the SC states.