In that case, you could try going into /ninja/resources/lang/en/ (or whatever other language you’re using) and editing texts.php. the view_invoice variable is on line 331. You should be able to change the string on the right side of the => to whatever you want (be sure to keep the single quotes), save the file, then the changes should take effect the next time that string is needed. I’ve only tested it by going into the template designer and seeing that the button changes there and in the preview, so YMMV.
Just be aware that changes like that won’t persist through an update.