Do you want the _exact_ book file you've downloaded or just _a_ file of the book you purchased (a license to read)?
For the latter, finding (main stream) ebooks isn't exactly hard, but it also isn't exactly legal.
For example I buy physical copies of books, which I don't have the space for. Then I donate them away and "acquire" the ebook version to read and archive. If an author sells the ebook directly without Amazon (or Rakuten) as a middleman, I purchase it from them directly.