change
$document_file = get_field('document_file', $post_id);
to
$document_file = get_attachment_file(get_field('document_file', $post_id, false));
setting the last parameter false for ACF gets the unformatted value, in this case the attachment ID. The WP function get_attachment_file() loads that file into the variable. Then you can pass this to convert it to text.
Then you call update_field() to update the text field with what is returned.
