set default print options on printer

[Gary Dale <garydale@torfree.net>]

I have a Samsung C410 printer attached to a Stretch server via USB and 
also available via wifi. There doesn't seem to be a default print 
setting for paper size but the cups listing for the printer shows:

   Defaults:job-sheets=none, none media=na_letter_8.5x11in sides=one-sided

which is correct. However when I try to print a PDF using Okular, the 
paper size is set to A4. If I forget to change it to letter, the print 
gets chopped off at the top.

Any ideas on what is causing this and how do I correct it?

Thanks.