Why PF3 doesnt obey octet rule?

PF3 does obey the octet rule.

Did you perhaps mean BF3?
BF3 is known as one of the few compounds that does not obey the octet rule, and is known even better as a compound that does not obey the octet rule because B has LESS than 8 electrons around it. More often, the octet rule is broken with there being extra electrons around the central atom.
The reason BF3 does not obey the octet rule is because there are simply not enough valence electrons. There are a total of 24 electrons. If B had 8 electrons around it, then one of the F's would not. F is very electronegative so you know that there is no way it would give up an electron to B.

PF3 does obey the octet rule. Its Lewis structure would have a bond from P to each of the F atoms, and one pair of unshared electrons on the P atom.

A similar molecule, PF5 does not obey the octet rule, though.