This will depend on lots of factors, including the type of materials in the archive, the geographic location, the size of the area, acceptable tolerance, temperature and volume of air exchange. Small mobile evaporative humidifiers are economic but have a low control accuracy, high maintenance and are not suited to large areas.
Electrode boiler steam humidifiers are a more professional solution, either as part of a ducted HVAC system or mounted directly in the room. They typically offer control accuracy of +/-7.5%RH. Resistive element steam humidifiers have a higher capital cost, but provide +/-3% control or better, and have lower spares and maintenance requirements.