How to change translation of some text on invoice?

Invoice language files are stored in site root/language/TAG/TAG.com_vminvoice3.ini.

You can change default translatable strings there.

Note that after each update or reinstall of component this language file is rewritten back, so you need to change it again. Because this, it is more recommended to use Joomla function Language Overrides function (Extensions -> Language Manager -> Overrides).

If you have enabled "Cache PDF", remember to click on "Reset Cache" button in General Config after you change some translation otherwise your cached PDF will still show old translation.