GENPDFADDON_
parameters can be overwritten in the final document either by setting preferences in WebPreferences or in the base topic (Ie. the one you are generating a PDF from): %ATTACHURL{topic="%WEB%.TopicName"}%
so you must expand the full path to attachments like this:
%PUBURL%/%WEB%/TWikiLogos/twikiRobot81x119.gif
Also, htmldoc must be able to download it so be sure to use %PUBURL%
not %PUBURLPATH%