You have to explain better what you are doing. You are in the Pages forum so I presume you are working on a Pages document which you later Export to Pdf, right?
You have to change the size of your document in Pages, which I wrote in my previous post.
Do you print at home? Then you don't have to export to PDF.
PDFs are really a kind of image format, used to communicate over platforms among other things. You don't do many or any change when you have saved to PDF. You do the changes in the applications you created the original document.