è molto semplice: la funzione non esiste.

Bisogna installare il modulo annesso per usare queste funzioni.

Dalla doc PHP:

Installazione
To get these functions to work, you have to compile PHP with --with-pdflib[=DIR]. DIR is the PDFlib base install directory, defaults to /usr/local. In addition you can specify the jpeg, tiff, and pnglibrary for PDFlib to use, which is optional for PDFlib 4.x. To do so add to your configure line the options --with-jpeg-dir[=DIR] --with-png-dir[=DIR] --with-tiff-dir[=DIR].

When using version 3.x of PDFlib, you should configure PDFlib with the option --enable-shared-pdflib