"Please note that the NCERT textbooks are copyrighted.
While copies of these textbooks may be downloaded and used as textbooks or for reference, republication is strictly prohibited. No agency or individual may make electronic or print copies of these books and redistribute them in any form whatsoever. Use of these online books as a part of digital content packages or software is also strictly prohibited. No website or online service is permitted to host these online textbooks.
Links may however be provided with written permission from the NCERT."
Now just select your Class, Subject, then the Book Title and finally click Go button.
Now you will able to see chapter wise list from the book on left hand side and at the bottom of that a link where you can download full text book as zip file.
In order to discourage piracy, the online textbooks that you would download may carry a watermark on all pages declaring the copyright of NCERT. So you should not buy or sell electronic or printed books with watermarked pages.
So this is how you can download NCERT text books online for free.