For some time now I have been using, or more accurately "not using", PDFZen for PDF annotation. There were certainly things about it that I liked, but it turned the pdf into an image to carry out the annotation. Highlighting was an overlay on the image itself rather than actual highlighting of the text.
Recently NotablePDF came across my screen. I tried it but I had issues with the highlighting. It felt clunky and there was only a choice of one color. I received an email from Hengjie, the Co-Founder, the next day. I shared with him my concerns and that I had already removed the app. Hengjie sent me an email today asking me to take another look.
Most of my concerns were addressed and the clunkiness was gone in the highlighting. Assuming you have aleady added the app and connected it to your Google Drive, this short video demonstrates the highlighting feature.
NotablePDF is by far the best annotation option avaalable for a cloud based or Chrome OS experience. The big winner for me is how it connects so naturally with Google Drive. I know any highlighting or annotations will be available next time I open the PDF in NotablePDF.
One future edition I would like to see is basic drawing tools such as with Techsmith Snagit. With that said, I am very happy with the tool and would recommend the app for Chromebooks where PDF highlighting is important.