StoredDocument.filename is the name the PDF is served under. It comes from options.filename on the render request, defaulting to document.pdf (or {key}.pdf for a batch item that supplied no filename). When you fetch the document from GET /api/documents/{id}/download, this value is echoed back in the Content-Disposition: attachment; filename="..." header, so it is what the browser or downstream product saves the file as. The filename is also returned in the store-mode render response alongside the documentId.
Skarn PDF
StoredDocument.filename
The download filename for a StoredDocument, set from options.filename at render time.