I recently experienced a problem on this front: I created a non-standard page size (half of a US legal sheet) so that I could do a 2up booklet on legal paper. Now Daniel mentioned above that the booklet feature defaults to the print size chosen. I tried to export the booklet as a PDF with legal size selected. While it will print just fine on legal paper, it will not export legal sized to PDF. The paper size is off in the PDF and when I try to print from the PDF outside of Dorico it results in very strange margins.