Your background image is not exactly high-res, this won't look great in print.
I don't know wkhtmltopdf itself, but your body already has absolute dimensions set (in inches). This is probably the problem. Your body has a max size, the content has an absolute size too (given due to the background image pixel dimensions).
This is not a good starting point for html-to-print transformaions, and PDF is essentially print.
what to do (intermediate)
- remove any size restrictions from body
- wkhtml... has a switch called
1.5should be an appropriate value to fill the page
- use page-size a4
what to do (the "right" way)
- remove size restrictions from body
- build the background borders (the black ones) with html elements and css styling
- refrain from defining "width" rules for those. You will only have to define a "width" once, all other widths should be set to "auto".
- heights will prove troublesome, because divs are only as high as their content requires. But setting height: 100% does not respect border and margin sizes.
- that yellow cross could be designed in css too, or a much higher resolution png/jpeg
- Use only "real" dimensions. That means do not use pixels, use points, inches, or mm. You can use % values, but make sure those are % values of real dimensions (that means that at some point a parent element has a real dimension)