Search for contacts with documents
It should be at url /civicrm/documents/search.

It looks like you need administer CiviCRM permission to use it.

If for some reason it isn't finding it try rebuilding the menu with /civicrm/menu/rebuild?reset=1

I'm assuming you mean this extension:
https://github.com/CiviCooP/org.civicoop.documents/blob/master/documents.php#L95