Actually, both are acceptable in this case. If you read the abbreviation using the words they stand for, i.e. "non-disclosure agreement", then "a" is correct.

Actually, both are acceptable in this case. If you read the abbreviation using the words they stand for, i.e. "non-disclosure agreement", then "a" is correct. If you read the abbreviation as letters, i.e. "N-D-A", then "an" is correct. Source: http://www.gpuss.co.uk/english_usage/a_or_an.htm http://www.gpuss.co.uk/english_usage/a_or_an.htm