When my app runs (or when I run from commandline) the wkhtmltopdf command I get the following error: (showing it from command line)
#> wkhtmltopdf wkhtmltopd
The Unix packages x11-libs/libXext and x11-libs/libXrender are dependencies
Gentoo:
sudo emerge libXext libXrender
PDFkit wiki also explains in more detail how to get wkhtmltopdf working on engine yard. PDFkit wiki
Debian or Ubuntu:
sudo apt-get install libxrender1
Hope this helps