How can I build a chatbot that answers questions only based on hundreds of large PDF files?

Hello everybody,
I have hundreds of PDF files containing valuable information, and I want to build a chatbot that answers questions exclusively based on the content of these PDFs.

I tried using ChatGPT, but I encountered a limitation: the maximum upload size allowed is 300 MB, whereas my files exceed this limit.

Is there a free solution or approach that allows me to work with larger PDF datasets and build a question-answering bot that only uses this data as its source?

Any recommendations on tools, libraries, or frameworks that support this kind of setup would be greatly appreciated!