Is it permissible to download and make changes to the books that are freely available on the internet and make money from them?
You should clarify if the books are copyrighted.
If you have downloaded, made changes and sold the books with the permission of the author. Though sometimes the author is ok with it but the actual copyright holder (who is not the author) is not. In this case, I think it would depend on the Islamic status of “copyright”. Should someone have the right to something they have not worked on? To buy “rights”. I don’t know the answer to that.
If you are “pirating”.
In the case of pirating, I recall a professor who pirated his own book and gave it for free to his/her students because he/she didn’t like the publisher/copyright holder. I don’t know if that’s halal. I am interested in knowing the answer