@KimLittle
What page size do you do for PDF? My experience with PDF has been about preparing material for printing so that fonts/images/format stays exactly the same. Do you set it to A5 or something?
PDF scales to the paper size you set at the time of printing. PDF was designed for that with smooth scaling: Portable Document Format. To keep proportions as much as possible you best create your document using the generic International ISO format, for example A4 or A5. It scales exactly to all other A formats and only minimally differs with other formats like legal. The actual printed size differs a little per printer since the printer firmware handles it a little different. Unless you know the exact targeted paper size A4/5 is your best bet. If you have no need to keep each page (length) exactly as created this should work perfectly fine.
For digital reading all bets are off because of the variety in screen sizes and display options of the PDF readers.