Hello, this issue has been solved on this question if it can help, you have to downgrade the wkhtmltopdf version : 

https://www.odoo.com/fr_FR/forum/aide-1/question/qweb-odoo-8-wkhtmltopdf-issue-with-ubuntu-14-04-64-bit-how-to-resolve-it-91342

Otherwise, you can try to add a fonction in this source code, that's how I resolved this problem -->

https://www.odoo.com/fr_FR/forum/aide-1/question/odoo-v8-0-odoo-forget-some-headers-and-footers-to-pdf-generation-103620