Don't scale the unit px to 1/2.54 points for PDFs

The unit px is one point inside the HTML source page but phantomjs handles it
without reason as 1/2.54 points. This makes the page smaller than expected when
trying to render a page as PDF.

https://github.com/ariya/phantomjs/issues/11590 ("page.paperSize is not
accurate for .pdf")
Disable page shrinking for pdf printing to create accurate output

PDFs are not rendered like PNG or other image formats by phantomjs because it
uses the printer functionality of Qt+Webkit. But Webkit uses some printer
"optimization" to save paper by shrinking the output. Such shrinking results in
too small content on a page.

https://github.com/ariya/phantomjs/issues/11590 ("page.paperSize is not
accurate for .pdf")