I’m using version 13.3 of NitroPDF. It cost me nothing to upgrade from PDF Pen Pro 13.1. However the website doesn’t list an upgrade price from older versions. And their subscription model doesn’t seem to apply to individuals.
I suspect that this is the end of the line for the product. I only use it for OCR, and it is invoked by a Hazel rule on incoming PDFs without the text layer. Everything else I want to do with a PDF is covered fine by Preview.