adding "pdf" to the relevant tabs in the homepage

I came to the homepage after a long time and it took some time to figure out where I can find the pdf version of the book. The image depicting pdf ? is overshadowed by the terms Mxnet and Pytorch. My ultimate tool, search in firefox, to find pdf in the page also didn’t helped. I would suggest it would help if you can kindly add the text “pdf” to the tabs because it will help non-ui-savvy person like me to quickly get to the pdf version.

Thanks and Regards,