When you go to Yandex, or Amazon, or Ebay... there is a search box you type what you want to type. If you long press that box, Opera pops up an option for you to create a search engine using that box. You can add Yandex as a search by that way.
Do you mean, name it to a constant name, so that only a single file will be stored, because there is a replace option? That's a very clever and useful idea, thank you:) Still, integrated pdf viewing would be quicker anyway. From now on it would't be as crucial, but I would appreciate if Opera could add it during their spare time:)