hi apsasia
there is no one size fit all solution for your problem.
the esiest thing to do is to resize the PDF on output.
in theory, the best solution is to recreate the file by moving the items from the A3 document to an A4 and resize each item (or group of items).
you can use the alt+ctrl(cmd) keys for scaling the content with the frame (does not always work in a reliable way, though... you have to try it out, if it helps in your case).
generally speaking, you will not layout the same elements in an A4 and in A3 document...
automatically resizing the whole document before exporting to pdf has in my little interest... it's easier and safer to resize the resulting pdf...