Here are ways to do it by simply adding files to an online interface. I mention them only because they are quite straightforward (and free) to set up.

- https://notebooklm.google/ (US or VPN): uses the "gemini pro" model.

- poe.com: You need to "create a new bot", disable "Make bot publicly accessible," and then "add a knowledge source." this offers many models, although the best ones require a subscription.