For the most part, Relevanssi Premium simply works with Document Library Pro. If you set Relevanssi to index the dlp_document post type, searching for document contents should work without issues.
If you want to enable Relevanssi in the search box above the Document Library Pro document lists, you can add this function to your site:
add_filter( 'document_library_pro_query_args', 'rlv_dlp_support' );
function rlv_dlp_support( array $args ) {
$args['relevanssi'] = true;
return $args;
}
This will enable Relevanssi in that search. This may have unwanted side effects in other searches on your site, so check that everything works after adding this.