Great question!
By direct we mean everything is done by PDFNet, with no dependency on anything else.
So indirect office conversion requires a 3rd party application that can print office documents. Ideally MS Office, but libre office is a possibility. Technically, on Windows OS, you can convert any printable file format to PDF using a PDFNet and an application that can print that file format to a printer, such as CAD drawings.
Direct conversion on the other hand, and PDFNet supports many, including image formats, is done entirely in PDFNet, and therefore works on any platform.
So using our new docx direct conversion, you can use PDFNet to view docx files on Android for instance. Or do docx conversions on cheaper Linux servers.
Our SDK Features page gives more details
https://www.pdftron.com/pdfnet/addons.html#Direct