Is it permissible for the owner of an engineering office to go to the offices of his competitors and offer his services to their clients, or to place advertising boards among their offices? And does this fall under the category of "receiving the caravans" [talaqqi al-rukban]?

There is no harm in offering services to customers in front of engineering offices and placing advertising billboards between their offices. As for the issue of receiving goods or caravans, it is unrelated to the question. It revolves around the merchants of the city going out to meet caravans and bring goods from the villages to sell them in the city market at a high price after buying them at a low price.