I don;t think there is any simple solution. Exporting, re-formating and re-importing is your best bet...especially if the format and layout is important. PDF is meant to be a static format, so changing the font size does funky things, even with re-flow enabled.
A couple of suggestions I'd give you is:
- Use landscape mode
- Use SoPDF to crop any whitespace on pages and to also cut a page into 2 pages. You can also use it to rotate the PDF. If you check the SoPDF thread, someone has released a GUI for it which makes it very easy to use
- If not SoPDF, try to use some software to cut out the margins and any unnecessary whitespace. It will increase the font size in the reader since it won't be wasting space displaying the margins.