That UX flow seems strange because the screen flow should just be: 1) upload, 2) pay, 3) chat. It shouldn't have asked you to check your email for anything.
Please feel free to email me at alfarez at gmail dot com if you're still seeing this issue.
As for handling large files, I'm using a popular method of breaking down the doc into small chunks and then just sending the relevant chunks to the AI.
How to pick the "relevant" chunks? Use embeddings and search for the embeddings that are "semantically closest" to the question being asked.
I wrote up an outline of this process here: https://farez.me/how-to-create-a-chatbot-with-openais-api-a-...