To build a chatbot that provides responses based on data in a knowledgebase of PDF documents, Azure AI Language custom question answering is the best option. This service allows you to quickly create a knowledge-based chatbot by extracting information from documents (such as PDFs) and answering questions based on that content. It minimizes development effort and costs since it provides an out-of-the-box solution for handling questions and generating answers based on unstructured data like PDFs.