Allways saves PDF as A4

[argotvisual]

In Ubuntu 18.04 it exports to PDF in A4 page size ignoring than the system wide default page size is Letter and the LC_PAPER is es_MX.UTF-8 (279x216mm)

[SokoloffA]

Paper size makes sense only when you send a document to the boomaga printer. At this moment your browser or other program split a solid document into pages
When document is exports to PDF, the boomaga cannot change the paper size.
So just change the paper size for the boomaga printer. Use the standard tools of your distribution for it.

P.S.
Now the program does not take into account the variable LC_PAPER, this is my fault. I 'll fix it in future releases.