To build a PDF analyzing bot with Haystack AI, Python, and WhatsApp, you need to set up a development environment, extract text from the PDF, use a predefined pipeline for question-answering, and integrate it with the Twilio Messaging API for WhatsApp. The process involves creating a virtual environment, installing required packages, building a pipeline using Haystack, extracting text from the PDF, and sending WhatsApp messages to users who interact with the bot. The bot can answer questions based on the extracted text and provide context to the user's queries. To test the application, you need to expose local endpoints over the internet using ngrok, set up a webhook on Twilio, and start the app.