{% comment %}
The PDF stage — what actually shows the document.

Default: the browser's own PDF viewer in an iframe. Shadow this template to
replace it; parody_web_annotate does, with a pdf.js canvas viewer it can draw
on.

It must REPLACE rather than overlay. An earlier design put a transparent
"annotation layer" div on top of this iframe, which cannot work: a page can
neither draw onto the browser's PDF plugin nor discover where its pages are.

Context: book, chapter, section, pdf_url, editions.
{% endcomment %}
<iframe title="{{ section.title|cut:'`' }} (PDF)"
        src="{{ pdf_url }}?inline=1#view=FitH"></iframe>