I agree that parallel port would likely be the simplest route. Serial coms has enough difficulties that you could spend your entire life on it.
I have used a USB-> Serial adapter quite a bit with the management of routers and they seem to work well. One issue is that they use weird serial port IDs. I just plugged one in and it has come up on Com15. Use a different USB port and I get a different port ID (Com17). ?You should be able to check the port ID with
Control Panel->Device Manager->Ports
Quite likely that a USB -> parallel port adapter does something similar. Check in the same place.
If you do choose serial you will need to set up the port in the Device Manager by right clicking and choosing properties.
Make sure that the plotter is basically working. If it doesn't have a display it is likely to have a way of stimulating some output, test page, configuration dump? You could post the exact printer model and maybe someone could check?
You can possibly (I am not sure if this will work in windows without permission issues) copy a test file to the plotter from a command window. Ideas on doing that in the link.
https://support.hp.com/gb-en/document/bpp02546