There's two main ways to "add documents to LLMs" - using documents in retrieval augmented generation (RAG) and training/finetuning models. I believe you can use RAG with Ollama, however Ollama doesn't do the training of models.
To index documents for RAG, Ollama also offers an embedding endpoint where you can use LLM models to generate embeddings, however AFAIK that is very inefficient. You'd usually want to use a much smaller embedding model like JINA v2[0], which are currently not supported by Ollama[1].
[0]: https://huggingface.co/jinaai/jina-embeddings-v2-base-en