Have you tried to directly saved the sent back html to local disk, then open it in another machine?
If this page can be opened standalone and the map works well, I think it can be saved by the 3rd part library.
But if it cannot be opened standalone, that's why you cannot export that as PDF.
What I suggest is export the map into image by our API in server side, you can view our sample project of Service Edition, which shows how to directly print data into an image layer by layer.
Wish that's helpful.