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, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
As long as the authors of these published books and the like permitted their use for free, then there is no harm in downloading and using them in study groups or for discussions among students or others because the owner of the book rights had made it accessible or has given up his right. However, if the author forbade the free publication of his books and retained his rights to his scientific efforts, then he alone has the right to utilize the books and benefit from the profits generated from printing and publishing them. This is not limited to this financial right, as there are other rights exclusively for the author, and they all fall under two categories:
moral (incorporeal) rights and material rights.
The moral or incorporeal rights are based upon a set of important principles, the most important of which are the following:
A. Establishing the authorship of the writer over his book and continuing to attribute it to him. The author does not have the right to give up his authorship of it to any other person or entity. It is also impermissible for anyone to arrogate someone else's book to himself by any means.
B. The author reserves the right of publishing his book and the right to control it after its publication. He may stop its circulation or printing if he changes the ideas or opinions that he expressed in it, for instance.
C. The author has the right to correct his work and modify it before the publisher prints a new edition of the book.
Islam clearly recognizes the incorporeal rights of the author. Dr. Bakr ibn ‘Abdullah Abu Zayd wrote, "The principles that protect the authorship of the writer and his book from alteration, modification, or addition and enable the author to guard his authorship and efforts fall into the category of the indisputably established facts and rulings of Islam. These principles have been clearly indicated in the universal principles and fundamentals of the Sharia, as has been highlighted in many past books such as Adaab Al-Mu'allifeen (Etiquette of Authors) and Kutub Al-Istilaah (Terminology Books)." [Fiqh An-Nawaazil 2/65]
The Council of the Islamic Fiqh Academy, in its Fifth session held in Kuwait in 1409 AH - 1988 AD, decided that copyrights and patent rights are recognized and protected by the Sharia. Their holders are entitled to freely dispose of them. These rights must not be violated. [Al-Iqtisad Al-Islami wal-Qadhaaya Al-Fiqhiyya Al-Mu'aasirah by Dr. ‘Ali As-Saaloos 2/748]
Hence, if an author allowed using his book free of charge, then there is no religious impediment to benefit of it, study it, copy it, and publish it for a purpose other than making a profit. If an author prohibited the free use of his book or allowed the free use of some of his books and excluded others, then only the books of which he authorized their free usage can be used.
Allah knows best.