The PF is a title which modifies the AVG (P
ER AVGVSTI, father of the Augusti), or perhaps a wholly separate set of titles (P
ELIX,pius and happy). In this case I assume it is the latter. In any case, they are almost never useful diagnostically due to their ubiquity, except for with rulers who were inconsistent in their usage (Gallienus, Carausius etc).
What is left appears to be -VS PF AVG, which is unfortunately common to all the possible issuers. However, we can see how long the legend is and it supports an attribution to Theodosius II, c.f. this example, also with the PF unusually far round the flan:
I think the bust style is too fine to be a later ruler, while the legend is too short to be Valentinian III.