¡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