This idea has been developed and deployed to Canvas

In the current beta, there is no method for rotating a document. Instructors cannot compensate for students who post PDFs in landscape format (for example). 

Please consider adding a button to rotate 90 degrees (left or right) to better enable instructors to allow alternate rendering and viewing of these documents.

Comments from Instructure

For more information, please read through the Canvas Release Notes (2018-10-06) .