Hii Yukesh!
There are a number of online portals which provide the options to download the books as a pdf format. You will get them when you search online. You can even get the Sarita Aggarwal books on the very famous and widely used e- shopping websites. You may search for the books in some Book shops. But, as you asked to download the book, I would suggest just go through the online portals and download the pdf. Hope you understand.
Good luck!