I was able to upgrade wkhtmltopdf to version 0.12.6 (with patched qt) by simply following this.
The bad news is that it did not fix my issue; when I print the Sales Orders etc to pdf, the company logo (letterhead) only appears on the first page and not on the following ones. This only happens if I access the webpage from the internet (typing erpnext.mydomain.com) whereas the logo issue does not happen if I access the webpage from inside the LAN where the web server is located i.e. http://192.168.x.x.
I have not installed SSL/TLS on the ERPNext server, instead I rely on my reverse proxy which has the SSL/TLS Certificate installed for the subdomain.
My next step will be to enable https on the erpnext server to see if it solves the issue.