PDFLIB PHP extension not found. You will not be able to generate PDF using 'PDFLIB' output method, as it requires PDFLIB PHP extension.

PDFLIB extension not installed on your computer Install PDFLIB extension yourself or ask your system administrator. Note that this extension can be loaded dynamically if your PHP configuration allows it.
Try using another PDF output methods
You have PDFLIB extension, but it is not loaded dynamically; you're getting warning message "Warning: dl(): Not supported in multithreaded Web servers - use extension statements in your php.ini in ..." You're using multithreaded Web server; in this case you should either add PDF extension to php.ini to be loaded statically, or switch to CGI version of PHP (or possibly change your HTTP server, if possible).
You have PDFLIB extension, but it is not loaded dynamically. No specific warning messages." Probably you have non-standard name of this extension; by default, the script searches for 'php_pdf.dll' on Windows and '' on *nix. Check if your extension have the same name.
You have PDFLIB PHP interface, but PDFLIB is missing or placed into directory where system cannot locate it; check if you have both parts of the PDFLIB package.