1. Make sure Adobe Acrobat Reader DC is installed.
2. Set your preferences for each browser you use, so that PDFs open with Adobe Reader
3. Open up your PDF from you library. It should give you the option to open in Reader.
4. Click the Comment tab to open the annotating and highlighting tools.
5. Click Save.
6. You'll be asked if you want to replace the file. Choose Yes.
*For Chrome, you may also have to make these additional browser settings: