Does mentioning volunteer work in one's CV—for the purpose of gaining employment—affect the reward for these deeds, and is it considered showing off?

The fundamental principle is that a Muslim should strive to conceal their good deeds and not display them to people, as this is more conducive to sincerity and closer to acceptance. However, if there is a prevailing benefit that necessitates speaking about the deed and disclosing it, then there is no harm in that, as long as the person does not intend by their discourse mere praise and commendation from people. Rather, they speak about it for the benefit that results from it. Therefore, refraining from documenting good deeds is preferable. However, if they are documented for a legitimate purpose without the intention of seeking people's praise, then this does not affect sincerity.