Is including sincere, God-intended charitable work from which one has received certificates of appreciation in one's CV for the purpose of job acceptance considered showing off?

It is permissible to mention a righteous deed performed seeking the Face of Allah if a legitimate benefit necessitates it. This is not considered showing off (riya') or seeking reputation (tasmee') unless that was the original intention. For example, the Companions used to strive in the way of Allah and acquire spoils of war without embarrassment, because their intention was not this worldly life.