How do I delete all highlighted text?

A script can be used to convert the highlight comments to redaction comments, and then apply them, thereby deleting the comments and all text underneath them.

Even though to the user it looks like the text and the highlight belong together, when you poke around in the guts of the PDF file, these are two completely different types of content, and therefore require two different appraoches: You first need to delete the hightlights, and then switch to the tool that allows you to edit PDF content. You will find this in the right hand pane under "Edit PDF". Select to "Edit" from the toolbar, and then you should be able to select text and delete it - that is unless you are dealing with a scanned page. And in this case, it gets a little more complicated, but we cross that bridge only if we have to.