How to access the dom made by the PDF.js and viewer.js

I am trying to have extension access the DOM after the two scripts PDF.js and viewer.js have finnished their converting of the PDF file to html. Check http://www.pdf995.com/samples/pdf.pdf and go into inspector to see what I example of what my extension will be working with.

I try to use eg document.head in my extension (in the content script) and it returns null. Obviosly nothing is in dom when I am calling it (but I am waiting to "run_at": "document_end").

I am obviosly missing some bit of knowledge here and am being led in circles by looking up stuff online