Are you saying that you intend to use GPT API for this instead of ChatGPT?
Different models have different limits
https://platform.openai.com/docs/models
Models with higher token limits also come with higher costs. Even if you use a model with a higher token limit, your PDF might still be too large for conversion.
It appears you have more tasks in mind beyond simply replacing words in the PDF and converting it to HTML. I’m currently uncertain about your specific needs.
If your sole objective is to convert a large PDF to HTML for viewing, you can refer to this post:
Link to PDF-to-HTML conversion guide
I’m providing this link to avoid cluttering this forum with references to commercial products.