The effects of ex situ vacuum and oxygen annealing treatments on thin films of the low-bandwidth compound Pr1−xCaxMnO3 (PCMO) are investigated. Structural and magnetic measurements reveal that increased ferromagnetism can be achieved by oxygen annealing treatment, which is linked to the increased Mn4+ ion content, as observed from x-ray photoelectron spectroscopy (XPS) measurements, as well as relaxation of the substrate-induced tensile strain of the PCMO unit cell. The increased number of Mn4+ ions and partial release of strain lead to stronger double-exchange interaction in the system. Vacuum annealing increases the ferromagnetic (FM) interaction as well; however, the increased FM ordering is not directly related to the improved double-exchange interaction, as XPS measurement reveals an indication of a slight increase in Mn3+ ions in this case. Trapping of carriers in the oxygen vacancies and formation of magnetic polarons have been suggested as the causes of the increase in ferromagnetic ordering, and this is also supported by the large coercivity and longer spin memory in the vacuum annealed PCMO.