Zend_Pdf module supports transparency handling.
Transparency may be set using Zend_Pdf_Page::setAlpha() method:
<?php
/**
* Set the transparency
*
* $alpha == 0 - transparent
* $alpha == 1 - opaque
*
* Transparency modes, supported by PDF:
* Normal (default), Multiply, Screen, Overlay, Darken, Lighten,
* ColorDodge, ColorBurn, HardLight, SoftLight, Difference, Exclusion
*
* @param float $alpha
* @param string $mode
* @throws Zend_Pdf_Exception
* @return Zend_Pdf_Page
*/
public function setAlpha($alpha, $mode = 'Normal');