Stroring the index in a format other than json

Hi all, I am using LayoutPDFReader to read multiple documents `doc = pdf_loader.read_pdf(pdf) documents.append(doc) ` Than create an index using the following code `for doc in documents: for chunk in doc.chunks(): index.insert(Document(text=chunk.to_context_text(), extra_info={}))` Now I need to store a perisistant copy of the index. Currently when using `index.storage_context.persist("Test")` the index is stored as a json file. Since one cannot use the _from_documents_ call (I get an error AttributeError: 'Document' object has no attribute 'get_doc_id') , how can I save the database in SQL format or any other format. Thanks