Hi @CynWinn-Lon,
Is the assignment in discussions, by any chance? So far, speedgrader does not offer docviewer for those assignments. The same thing happens when an assignment is "text entry": no docviewer available for either teachers or students. In fact, I don't think there is an option to turn the docviewer on or off yourself.
The best option is to let the students upload their assignment as a document (pdf, doc, odt) file. I've done this before, and my students have been able to drop pins and highlight in their assigned peer reviews. (Additionally, some of my students now also use it to reply to my feedback pins/highlights if they have additional, specific questions). Make sure to mention it only works in that format, though, because I got quite a few questions about "what happened to the pins" once we started working with peer reviews in discussions 😉
I hope this helps!