Notary Question

I am a notary using a MacBook Air laptop computer,  Operating System: Mojave, Version 10.14.3.

My printer is a Brother MFC-L5800DW with dual trays for printing both legal and letter sized pages from large PDF files.

I do this using Adobe Acrobat Reader, in the Print box, under Page Sizing and Handling, by checking the box: "Choose paper source by PDF size".

There are situations wherein I want to print a second copy of the same file, but this time on "letter size" paper only.

I do not see how to switch from "Choose paper source by PDF size" to "letter size" only.

Please advise.

Thanks.

Lloyd D