This idea has been developed and deployed to Canvas
For document previews using the DocViewer tool, there really needs to be a full-screen option (this feature was available with the Box previewer).
In addition to normal file previews, it would be great if the full-screen option was also available for students when they review the annotations on their submissions. Right now, when students review their submissions, they get a window-within-a-window view of their annotated submission. It would be a better user experience if they could throw that preview full-screen to review the feedback and comments from their teacher. Again, this feature was available for previews with the Box previewer.