Add , the most general word in this group, means to join or unite so as to increase the importance, size, quantity or scope of something: to add a new line of merchandise to one’s goods; to add a new wing to a building; to add five new salesmen to a staff; to add a touch of levity to an otherwise solemn speed.

Attach , as here considered, means to connect or join on as a part, and is close in some contexts to append : to attach a stipulation to a contract; to append a query to a manuscript. Append emphasizes that the addition is subordinate or minor in relation to the original work. Both words are formal, but attach has a legalistic ring to it lacking in append : to attach a rider to a bill; to append a footnote. Note that add could be used in place of wither of these words, but would make the tone less formal and therefore less impressive.

Affix means to fix or attach to: to affix a seal to a document. Affix is appropriate only in very formal contexts, as in the description of state affairs: The Govenor-General affixed his signature to the bill regulating assisted immigration.

Annex means to add something as a supplement. It implies not only that the addition is a subordinate part, but often that the addition remains distinct: to annex an adjoining territory; to annex a building to an older one.