PFI pins are just terminals. Tasks can use them to either import or export signals, but they have no independent capability of performing tasks (like PWM) on their own.
Whatever counters you have on board can be used to generate PWM, and you can then *route* the output signal to whatever PFI pin you choose, but it'll be the counter task that owns the job of PWM generation.
(The spec sheet for your device says it has 4 counters. Still not quite enough, but closer...)
CAUTION! New LabVIEW adopters -- it's too late for me, but you *can* save yourself. The new subscription policy for LabVIEW puts NI's hand in your wallet for the rest of your working life. Are you sure you're *that* dedicated to LabVIEW? (Summary of my reasons in this post, part of a voluminous thread of mostly complaints starting here).