Many e-book stores offer genuine e-books in the PDF-format. Here are some common purchasing channels: 1. Amazon's e-book store: Amazon's e-book store is one of the most popular e-book stores, providing a wealth of genuine e-book resources in the form of PDF-s. You can search and buy it on Amazon. 2. Jian Shu: Jian Shu was a popular online literature platform that also provided a large number of e-book resources in the format of PDF. He could search and purchase them in the book. 3. Douban Reading: Douban Reading is an e-book platform on Douban that provides a large number of genuine e-book resources in the format of PDF. You can search and buy it on Douban. 4. Other online e-book stores: e-books stores such as iBooks, Google Play Books, and Barney & Noble also provide a large number of legitimate e-book resources in the form of PDF-s that can be searched and purchased in these stores. Please note that there is a certain fee for purchasing an e-book in genuine PDF-format. It is recommended to carefully compare the price and resources before purchasing and choose a reliable purchase channel.