Is it permissible to benefit from books in PDF format sent by a professor, knowing that the copyright is reserved for a specific entity, or must they be purchased?

Intellectual property rights are protected by Sharīʿah, and it is not permissible to use or distribute copyrighted books without the permission of their owners. If the prohibition applies only to printed books, then it is permissible to use them in PDF format.

However, if the prohibition includes all forms (including PDF), then one must consider how the professor obtained them: - If it is highly probable that he obtained them through illicit means, then it is not permissible to benefit from them or copy them. - If it is highly probable that he obtained them with permission, then it is permissible to benefit from them, as the default is to assume good intentions on the part of a Muslim. - If the situation is unknown, then the unknown is like the non-existent.

Personal use may differ from commercial use in terms of permissibility of benefit.