These functions are used to prepare PDFs as input to the chatbot. The
content_pdf_url() function is used to provide a URL to an PDF file,
while content_pdf_file() is used to for local PDF files.
Not all providers support PDF input, so check the documentation for the provider you are using.
Both functions embed the PDF's contents in every request, so for a large
PDF, or one you'll refer to across several turns, prefer
chat$file_upload(). It uploads the file once and later turns reference
it by id.