After inserting the frappe.throw(html), when you try to create a pdf, it will show an error instead which contains the html code of the pdf to be generated:
Right click on this and select “View Source Code”:
Then, look for the inner html, which is the actual html that would go to wkhtmltopdf, it is where <!DOCTYPE HTML><html>...</html> are found:
Extract this to a new file, copy to the server and process with wkhtmltopdf. This should give you a much more detailed error trace.