What library is used for PDF in PHP?

PDFlib version 6 may be used to produce PDF files using PHP’s PDF methods. In addition to the function-oriented API for PHP4, PDFlib provides an object-oriented API for PHP5.
There’s also the Panda module to consider.

FPDF is a PHP class that allows you to create PDF files using only PHP (without using PDFlib). The F in FPDF stands for Free, which means we may use it for any purpose and adapt it to fit our requirements. FPDF works with PHP4 and PHP5 and requires no extensions (besides zlib to enable compression and GD for GIF support).