Ok needed to download next -pre version of asciidoctor-pdf which looked like this:
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jnorthr@jnorthr-EC548AA-ABU-a1128-uk:~/Dropbox/Asciidoctor/HarvesterProject/asciidocs$ sudo gem install asciidoctor-pdf --pre
[sudo] password for jnorthr:
Fetching: asciidoctor-pdf-1.5.0.alpha.6.gem (100%)
Successfully installed asciidoctor-pdf-1.5.0.alpha.6
Parsing documentation for asciidoctor-pdf-1.5.0.alpha.6
Installing ri documentation for asciidoctor-pdf-1.5.0.alpha.6
Done installing documentation for asciidoctor-pdf after 3 seconds
1 gem installed
jnorthr@jnorthr-EC548AA-ABU-a1128-uk:~/Dropbox/Asciidoctor/HarvesterProject/asciidocs$ asciidoctor-pdf -v
Asciidoctor 1.5.0 [http://asciidoctor.org]
Runtime Environment (ruby 2.1.2p95 (2014-05-08) [i386-linux-gnu])
jnorthr@jnorthr-EC548AA-ABU-a1128-uk:~/Dropbox/Asciidoctor/HarvesterProject/asciidocs$ asciidoctor-pdf Notes.adoc
asciidoctor: WARNING: conversion missing in backend pdf for pass
asciidoctor: WARNING: conversion missing in backend pdf for inline_image
Failed to parse formatted text: <button id="chooser-demo" onclick="changetext(this)">Click me</button>
asciidoctor: WARNING: conversion missing in backend pdf for pass
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The warnings mean something to someone, just not to me. The .adoc code for 'Click Me' is:
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== Button Test
IMPORTANT: Can only run this test within Tomcat, etc.
Ok, here, we try a button to open a chooser:
+++<button id="chooser-demo" onclick="changetext(this)">Click me</button> +++
'''
but it runs thru asciidoctor fine without a burp so dunno :-}