In the latest version (18.8.5), you can now retrieve PDF using API.
Let’s say you want to generate PDF for sales invoice. Have a look at the URL of invoice view which would be something like this…
/sales-invoice-view?Key=c738c447-d700-430b-9956-010046a32776&FileID=5fa5dd21-0ca9-4672-b242-486895c40f5e
to get PDF, just add .pdf extension to /sales-invoice-view resource so it’s:
/sales-invoice-view.pdf?Key=c738c447...