First, the virtual printer is not actually required for Office conversion, if you have MS Office installed. If MS Office is installed, PDFNet will use Interop to communicate. RequiresPrinter returns true for office documents, though it is really optional for office documents.
The virtual printer is typically used for other file types, e.g. AutoCad DWG.
If you don't have MS office installed, and want to convert office documents using something like LibreOffice, then yes you would need the virtual printer.
Also, you only need to install the printer once. Not every time you want to convert.
To install the virtual printer, you need to be running with elevated rights, and PDFNet 32bit|64bit needs to match your OS 32bit|64bit.