How to change name of the viewer folder?

Put a file .htaccess inside the root of the application with this content (change new_viewer_name with desired name):

<ifmodule mod_rewrite.c="">
    RewriteEngine On
    RewriteBase /
    RewriteCond %{REQUEST_URI} ^/([^/]+)
    RewriteRule ^new_viewer_name/(.*)$ /%1/viewer/$1 [L]
</ifmodule>