Unable to change paper size to A4

LO Version: Writer
IMac 2.9 GHz Intel Core i5
8 GB 1600 MHz DDR3NVIDIA GeForce GT 750M 1024 MB
I am trying to produce a 40 page A5 booklet on A4 landscape paper to output as a PDF spread for commercial printing.
I can successfully achieve all I need until I reach the stage of exporting to PDF, I am then unable to set an A4 paper as the program defaults to US Legal and cannot be changed. I have set the Language to English UK. Is this a limitation of the program or am I missing something?
I have recently migrated from Windows to a Mac and I am trying to use Libreoffice to replace MS Publisher. So far I am very impressed with Libreoffice but I am very surprised by this problem. Please tell me where I am going wrong.

In Windows there is no option to set the paper size - it uses the paper size set in the document. You could try a program such as Nitro PDF which in effect installs a printer which then prints to a PDF file.

There are two ways off the top of my head. Print to a printer that can create the booklet. Or. Do the paginating after export to PDF. There are free apps to turn your A5 pages into a booklet.