In Spain it is quite common to use the hypocoristic Paco to refer to the people with the name Francisco.
Why is that? Wikipedia states that:
In Spanish, people with the name Francisco sometimes are nicknamed "Paco": San Francisco de Asís was known as Pater Comunitatis (The Community father) when he founded the Franciscan order, "Paco" is a short form of "Pater Comunitatis".
But there is no reference to this fact and I wonder if this is another built-up story related to ¿Por qué Pepe es equivalente a José? with Pater Putativus.
Note, as seen in comments, that also Pacho (in Colombia) and Pancho (in Chile, among others) are used as hypocoristics to Francisco.