Adobe Printer's default setting changes back to "letter" instead of staying A4

I have exactly the same problem with Adobe Acrobat Reader (64-bit) defaulting to US letter size rather than A4. My printer is set to use A4 paper as I am in the UK and all setting reflect this.

The instructions to change the Printing Preferences of “Adobe PDF” under Printers & Scanners fails because “Adobe PDF” is not an available option.

Clearly Adobe Acrobat Reader (64-bit) has a default of US letter size built in to it. Where exactly is it and how do you change it?

​@Johan_Massyn To make A4 the permanent and go-to-default-size, you have to change it at the "system" level, not the "app" level. Here is the move:
 

1: Printing Preferences (Search Menu > Printers & Scanners > Adobe PDF > Printing Preferences > Adobe PDF Settings Tab). Change "Adobe PDF Page Size" to A4.

2: Advanced Settings Inside that same Preferences window, click the Advanced button. Under "Paper/Output," ensure the "Paper Size" is also set to A4.

For more details, https://helpx.adobe.com/acrobat/desktop/print-documents/set-up-and-print-pdfs/pdf-printer-settings.html and https://helpx.adobe.com/acrobat/desktop/print-documents/set-up-and-print-pdfs/page-size.html