These might be of help:
- fpdf - PHP class-based generator
- wkhtmltopdf - Convert html to pdf using webkit (qtwebkit)
- mPDF - PHP class which generates PDF files from UTF-8 encoded HTML
- TCPDF - an enhanced and maintained version of fpdf
- dompdf - (mostly) CSS 2.1 compliant HTML layout and rendering engine
- Zend_Pdf - A PDF manipulation engine/component from Zend
- PrinceXML - Commercial, but supports CSS3 transforms
It's partly dependent on how complex the HTML is and what you want to display. Some of them can be a bit flaky when rendering certain style-based elements (especially CSS2/CSS3), but in general they are pretty good.
If you're planning to "force-download" your PDFs, then you'll need to use something like the following in PHP:
header('Content-disposition: attachment; filename=filename.pdf');
A good example/explanation can be found in a previous SO answer: php: force download to harddrive?