Add product description to item name

gistfile1.txt

add_filter( 'pdf_invoice_item_name', 'add_product_description_pdf_invoice_item_name', 10, 4 );

function add_product_description_pdf_invoice_item_name( $item_name, $item, $product, $order ) {
  // Use $product->get_id() if you want to get the post id for the product.
  $item_name .= '<p>' . $product->get_description() . '</p>';
  return $item_name;
}