On large screens open book link will display book embedded in the page. On smaller devices “open book in new tab” button will open pdf in a new tab, because iOS

On large screens open book link will display book embedded in the page. On smaller devices “open book in new tab” button will open pdf in a new tab, because iOS Safari is limited with embedded pdf, with this button book is going to be opened in a separate tab where you can either download it to your device or read it from browser.