Quick note. If you’re having problems generating PDFs with wkhtmltopdf and it’s giving you an error that says it has failed without an error message, make sure you have a good copy of wkhtmltopdf on your server. In my case I had uploaded the file with FTP, and used Filezilla’s auto mode.
Apparently Filezilla didn’t know how to transfer the file, and it screwed it up. In my case I had to manually transfer the file in binary mode, and as soon as I did that it started working.