¡La Esquina de la Gramática! Rule 4
Acronyms plurals…not the same as English. In Spanish acronyms don’t change when written in plural. The plural is determined by the words that introduce or modify them. For example: Varias ONG europeas Unos DVD Los PC It is incorrect to use the English rule: PC’s ONG’s PCs ONGs. Source: http://www.rae.es