¡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