DOCBOOK: Why cant i use format pdf in imagedata??

I am using docbook for producing websites. In docbook there are about 
30 different formats for the attribute format of an imagedata tag. 

Why cant i use PDF as a format? PDF for me is an imageformat like ps.
or should i use <ulink> or <olink> to reference an existing pdf 
document??  

any suggestions are appreciated

regards,
janning