Generally, to initial something is similar to signing something, but does not need a whole name, just the first letters, so John Smith becomes JS, hand written like a signature. So, often for an imprtant document, you have to sign at the end, but initial every page, so no page can be swapped. . A tax invoice is often part of a receipt when you buy something in a place that collects taxes on purchases, which is likely almost everywhere. . So initials the tax invoice? approve, agree, present tense? He initials the tax invoice.

Context?