Your instructor may choose to include comments directly in an assignment  submission using inline comments, or annotations. If an assignment's  submission details page includes a View Feedback button, you can view these annotated comments through a preview tool called Canvas DocViewer. You can also use Canvas DocViewer to reply to any comments or make your own annotations. 

Some browsers may not initially render comments correctly when viewing  feedback. If your browser includes a built-in PDF viewer, select the  option to view the PDF in the system viewer. You can also download PDF  files with comments to view on your computer. The settings required to  view or print the annotations in the PDF vary based on the software  installed on your computer. 

Note: If the assignment attachment displays a Preview button instead of the View Feedback button, your file is not DocViewer compatible and will not include any annotated comments.