"F" fäng gesprochen mit einem Vokal an: f = eff

also:
an eff die ei

Danke!!!

It depends on how you expect the thing to be pronounced. I wouldn't be sure whether to write "a FAQ" or "an FAQ", for example, because I'm not sure how the reader would read FAQ.

escoville, FAQ may be pronunced F A Q or F*CK, but FDI can only be pronouncedd F D I, not "feedee"