Responding to this issue, Dr. Monzer Kahf, Scholar in Islamic Economics & Financial Expert, states,

When you download from a web that allows it free, you have the right to download and use for personal not for commercial use. If there is an issue of copyright, it is a matter of concern of the owner of that website. But most likely these books are made free for download most of them are financed by government or charitable institutions that aim at providing them free of charge; there are a lot more on the internet for free downloading. I think you should feel comfortable with taking your copies. A worry on copyrights comes if you download materials that is not normally allowing it and you use sophisticated method to break their codes.