PDF Printers don't really have a paper tray/feed, so this would need to be configured and will surely default to A4 or Letter. Designing a report for a paper size doesn't tell the printer to feed that size, no, that's not the way it works.
Since printable area is very printer specific, designing for that option means designing for the active printer. Not, what you want. Design for whole page and have large enough margins to print within a reasonable area. With many printers, especially inkjet, you may print almost 100% of a page, but it depends. Documents are better readable with margins anyway.
I know it can become very cumbersome to get a report working. For example it took me some time to understand Zebra printers will measure ribbin width as page width, even if the label roll is narrower. In that case still the best mode was to give the report a margin I wanted on label plus the distance to the ribbon edge.
Bye, Olaf.