In the asciidoc there is an image that I'd like to include only for PDF output. Is there a kind of attribute I can pass to image:: so that the image is processed for PDF generation, and ignored when generating epubs etc.? Or more probably by using ifdef, but how exactly?
You can use the ifdef directive like this:
Some text for all outputs.
ifdef::backend-pdf[]
This is only displayed in the PDF document, you can use image:
image::mypict.png[]
endif::[]