You have quite many sources to get Hindi PDF books. Some of them are listed below -
This is a bookstall available online. You can find a huge list of Hindi books on it. The best part with it is the variety you can find in terms of categories and genres.
This is yet another good source for Hindi PDF books. It has almost all top Hindi writers in its collection. The books are free of cost. Visit http://www.hindiebooks.ga/ for more info.
DailyHunt, previously called NewsHunt has an interesting collection of books in Indian languages. The service has a website as well as a mobile app. The store offers books both in free and paid categories.
The site can be accessed by visiting the link http://pdfbooks.ourhindi.com/. The site lists a lot of books in Hindi.
If you are specifically interested in the books authored by Khushwant Singh, you may check out Flipkart, or Amazon.
The site lists a lot of books authored by the renowned writer. You can access it by visiting the specific section at http://www.flipkart.com/author/khushwant-singh. I could find that the books are priced competitively. But, I do not think you can get any books in PDF form over there.
Apart from the ones mentioned above, there are many sources to get the ebooks. Rather than going by the book store, I would advice you to search based on your choice of books. Exactly the way we choose an online store when we need to purchase a gadget.
As for the book Bhaang Ki Pakodi mentioned by you, I could not locate any source for the same.