We have some instructors with password protected files, such as PDF files, Word Documents, and I'm sure at some point will be Excel files. Some times there is rationale for adding an extra layer here, but currently (at least with PDF or Word files) the DocViewer does not support password protected files. You just get an"Error Loading Document" (below).
With Box, if a PW protected PDF was enabled, when previewed, the rendered image would prompt the user for a password as follows even if embedded as a widget.
I'm sure this is likely not a high priority type of request, but thought I'd submit and see if I could get anyone to vote for having the doc viewer support password enabled files as well.