how to annotate scanned PDF on MAC
Highlighting text in a PDF assumes that there is text in the PDF to begin with.

When you scan a document to PDF, it is simply an image. You should still be able to use the annotation tools available in preview to draw on it and add notes, but you will not be able to highlight or copy text from it because there literally is no text in the document.

There is, however, a type of software called OCR, for Optical Character Recognition, that can be used to programmatically "read" the letters in the image of your document and write out accurate text to the PDF. Some of this software is pretty good, but it is rarely free. Some scanners come with OCR software, and the you may be able to find some software online that will let you try it out for free.

Running an image PDF through OCR software will provide it with text data that you can then highlight using the annotation tools in preview.