Hi Nienke,
If your pdf has both A4 and A3 sized documents and you want to print them on sheets matching the page size, you have to look for the following things:
1) Your target printer must be capable of handling both the page sizes.
2) You must select "Choose paper Source by PDF Page size" option in the main print dilaog of Adobe Reader.
This option lets the printer choose the page size as per the document size.
Regards,
Rishi