This » PECL extension is not bundled with PHP. Information for installing this PECL extension may be found in the manual chapter titled Installation of PECL extensions. Additional information such as new releases, downloads, source files, maintainer information, and a CHANGELOG, can be located here: » http://pecl.php.net/package/pdflib.
To get these functions to work in PHP < 4.3.9, you have to compile PHP with --with-pdflib[=DIR]. DIR is the PDFlib base install directory, defaults to /usr/local.