http://www.crocodocs.com has become my workhorse site for teaching. I use the site almost daily now. Crocodocs allows the user to annotate a pdf by drawing, adding text, and highlighting. The user uploads either a pdf file or a docx file, which the site converts to pdf. The site is set up for business users, but it provides free personal accounts for educators.
I use the site for handouts that I use in class. Either I or a student annotates the handout by putting in the correct (we hope) information. The document is automatically saved and available for future use. I give the students access to the class folder so they can access it after class for review or for an exercise that we didn’t finish in class that I completed for them.
The application is not perfect. I like the different annotation tools even if they are bare bones, and they have improved over the year I have been using the site. However, if I share a document or folder with the students, they can edit it later which means sometimes students change an annotation resulting in incorrect answers. It doesn’t happen much. I would like the site to allow the site account to allow sharing without enabling editing for everyone like Google documents does.
That being said, the site has proven very useful with my use of the flipped class model. We can review in class as the work progresses. I got extra encouragement when we reviewed a test the students had taken because the explanations made more sense to them as we reviewed the troublesome items.