All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
There is no harm in downloading a book that is online if there is nothing that prevents from downloading and reading it.
Some scholars are even of the view that there is absolutely no sin if one uses it for a personal benefit without earning any money from it.
Personal benefit is if someone wants the book to read and not to sell it or earn from it.
Therefore, there is no harm at all in benefitting from the mentioned book, especially if there is nothing that indicates the prohibition of downloading it.
For more benefit, please refer to Fatwa 394327.
Allah knows best.