Facebook and once in email, I have been asked about how to annotate PDFs. In both instances my response was to take a look at using Kami to annotate PDFs. Kami is a browser-based service through which you can draw, highlight, and type on a PDF. You can share your PDFs in Kami and write notes in the margins for others to see and they can do the same. Watch my video that is embedded below to learn how to annotate PDFs by using Kami.
Disclosure: Kami is an advertiser on FreeTech4Teachers.com