This Bengali book for secondary course (X) is not available in NIOS website as a soft copy. You can download scanned version of this book in PDF format. The scan quality is not so good but readable. Hope you will find it useful. Check the following links to download the book in PDF format.
NIOS Bengali book for class X Part_1 : https://goo.gl/w1YtGw NIOS Bengali book for class X Part_2 : https://goo.gl/fgX5oF