Hello Srishti,

You can download the official NCERT Class 8 Mathematics textbook free of cost from the NCERT website (https://ncert.nic.in/) . But reference books based on NCERT and not the original textbooks are published by Dhanpat Rai & Co. Those books (such as by R.D. Sharma or others) are typically available on websites such as Amazon, Flipkart, or Google Books but not free of cost for legal download.

If you need additional practice material from Dhanpat Rai & Co., it is better to purchase the printed version or see if your school has circulated a PDF version.