could not generate pdf for wkhtmltopdf '/var/www/html/wp-content/uploads/2016/04/indicator_1461952036.2848.html' '/var/www/html/wp-content/uploads/2016/04/indicator_1461952036.2848.pdf' --page-size 'Letter' --orientation 'Portrait' --footer-spacing 3 --header-spacing 3 --margin-top 20mm --margin-right 20mm --margin-bottom 20mm --margin-left 20mm --footer-html 'http://www.childtrends.org/wp-content/themes/childtrends/pdf_generation/pdf_footer.html'