A page in a PDF has an overall size and may also have a trim size. To see the trim size of a page, go to Acrobat preferences> General> show Art, Trim and Bleed boxes. Or go to Tools> Print Production> Set page boxes> trim box. I use Impostion software to do what you are doing, but I think many people simply place their PDF(s) into InDesign and use step and repeat to impose their press sheet. When you place a PDF into InDesign you can choose to crop the PDF to the trim, bleed, or overall size.
As for your noise issue, I'm thinking you are printing your PDF as an image, which is converting your nice vector art into a raster image, the same thing happens when you open a PDF in Photoshop.