Highlight words in Document Viewer
Solution 1:
Document Viewer DOES actually support annotations and highlightings in PDF files (not in other formats). Just click on that notebook like button and you'll be able to annotate and highlight text.
Solution 2:
Try Mendeley (https://www.mendeley.com/)! It does the work like a charm!
In fact, Mendeley is more than a PDF-reader. You can use it to organize your docs and references.