The thing is huge but it beats the crap out of most things out there. I'm talking about tufat.com's html2ps and html2pdf PHP library. Check it out here:
http://www.tufat.com/s_html2ps_html2pdf.htm

I took the HTML output from Ubercart's invoice system and fed it to this library and, voila, PDF invoices. Lovely stuff :) Especially when you consider that I've been struggling with this PDF stuff all day long.

Make sure you give html2Fpdf a skip. It's quite a waste of time. Enjoy!