It’s not very straightforward to directly upload pdfs and run inference/finetune model from their contents. The reason being that pdfs can contain all kinds of data (images, tables, latex etc) and it all has to be normalized and cleaned.
Take a look at the haystack project. It is a toolkit for building pipelines for q&a, search, qg etc tasks and it’s fairly easy to get started. They have a node specifically for using GPT3 for the answer generation.