Sounds to me like you are not "Publishing" your layout pages.
I am guessing you are "Printing" them - (i.e. File menu > Print)
When you set up the Publisher and set that to "Save" as a PDF then it will use the built in PDF converter and will use the Master page size as your PDF page size automatically.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11