Take a look at wkhtmltopdf, for which support is built-in to Tiki
Mod mozilla2ps
mozilla2ps is a XUL application to transform HTML (with CSS) into PDF format. It requires the following applications to be installed on the server: xulrunner (to run XUL applications), Xvfb (or another X server) on display :1 or xvfb-run, ps2pdf, and some basic fonts to be used by the X server. This mod supports everything that firefox supports and will generate a PDF that really looks like what the user sees in it's browser, even with complex Cascading Style Sheets. It's also faster than html2pdf or dompdf