After clicking the download button, you will enter google drive to read the book online. At the very top of the page there is a horizontal toolbar. There you will find a symbol of 'download'. Click it to download.
If you don't find it there, you can find a collapsable option there on the toolbar. Click on it and you will find the download option.
If you are using a laptop or desktop you can simply press (ctrl+s). If you have a smartphone, download HACKER'S KEYBOARD and from there you can tap (ctrl+s) to download.