wkhtmltopdf is an open source (LGPLv3) command line tool (Simple shell utility) to render(convert) HTML into PDF using the QT Webkit rendering engine. It runs entirely "headless" and do not require a display or display service. There is also a C library.
Source: http://wkhtmltopdf.org
standard: fewer features, requires X (and if your GNU/Linux OS is virtualized, it perhaps won't work)
patched QT: more features, doesn't require X (headless)
OpenHub
Configure
To configure with Tiki, go to General » General Settings admin panel (tiki-admin.php?page=general&cookietab=2) and set "PDF from URL" to "WebKit (wkhtmltopdf)". Then in the "WebKit path" insert for example this full path to the executable file: /var/local/bin/wkhtmltopdf-amd64 (on a Linux 64-bit system, the path can be different depending on the installation).