Hosting a directory of legacy static HTML files on Drupal
As long as you're using the default rewrite rules you don't need to do anything.

Drupal's .htaccess file contains this:

# Pass all requests not referring directly to files in the filesystem to
# index.php. Clean URLs are handled in drupal_environment_initialize().
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !=/favicon.ico
RewriteRule ^ index.php [L]


So requests for existing files won't be re-written, the file will just be served.