I was able to fix the issue. After granting authority to the Guest user, the download servlet now works! Thanks for the help.
On a somewhat related note, when a user tries to access a file as Guest, and Guest does not have permission to view, it sends the user to Guest Home in the web client. Is there a way to display an error message "Access denied." instead of forwarding to Guest Home? I am going to start looking in the source to see if I need to write an aspect or something. Is there a better way to do this?