Rater to pdf poor quality

I am using pdf printer driver with minor customisation. All settings are maxed out. The background raster output in the pdf file is pixelated. 

Oddly when printing sample (smaller area - A3 size) the output looks much better. Only when switching to whole sheet (custom paper size over A1 size) 

the screen capture from sheet model view:

screenshot of corresponding area from pdf

pdf driver configuration: