Given that the current invoice, shipment and credit memo PDFs look the same as in Magento 1 and PDF's are always a bit of a nightmare to customize for the merchants:
I suggest to replace the PDFs with a html2pdf tool. Actually the preview of the invoice looks already really nice in the backend, so this could just be converted to pdf.
This would maybe also help the problem Hirokazu Nishi mentioned in his tweet:
"Magento2 still use LinLibertine font. It’s only for Alphabets. To change PDF font, we still have to rewrite some classes..."
This idea resulted from a tweet/discussion here: