Interesting grammar point.
A NDA is correct because it's a non-disclosure etc
But we want to say "an en dee ay" because of the vowel _sound_.
C.f. A European Union citizen, an Eeee you citizen. (Also vowel _sounds_ A yooniversity. An uhmbrella. An X (ecks) Ray.)
Hope that helps!
@Homebrewandhacking Huh, isn't the sound in pronunciation what decides whether 'a' or 'an' is used?
I was always under the impression that this is an optimization to make it easier to both pronounce and understand…
@dramypsyd