As far as I know, iAnnotate software doesn’t have a way to automatically highlight all your search results.
However, good ole MS Word — even my 2004 copy for Mac — has that option under Find and Replace. This article has instructions.
In addition to or instead of highlighting the text, the Find and Replace formatting box lets you make font changes like underline and bold to the search results. You can even change the font, its size, and color on each search result.
Of course, you also can change the font type and size for the