The Highlight tool won’t work in the following situations:
1. The document has restrictions allowing no comment on it. To check the document restrictions, navigate to File > Properties > Security.
The document doesn’t have searchable text, it is an image or
scan-based. To check if the document has searchable text, navigate to View > Text Viewer
to see if there is any text there. If no text appears, that indicates
the document is a scan-based document. You can try one of the following
1) Use the OCR (Optical Character Recognition) feature in Foxit PhantomPDF to make the text recognizable and highlightable.
2) Use the Area Highlight tool under Comment menu.
I cannot Highlight and Edit on a standard document