What is the ruling on working for a pollution control company in a European country, which involves commissioning and maintaining machines placed in factories that produce prohibited materials such as magazines or posters related to alcohol and cigarettes?

Islamic Sharia prohibits whatever leads to the unlawful, and cooperation in sin and aggression is forbidden. Your supervision of machines used in factories that produce or promote the unlawful is considered assisting in sin. If you can limit yourself to companies that do not deal in unlawful matters, then there is no harm; otherwise, seek a job free from the unlawful. However, if the function of the machines is solely to prevent environmental pollution, then working with them may be permissible.