PDF version 1.7 by default?

[Karl Berry]

    Admittedly Ukrainian printers may use older equipment and software.

>From what I read online, it is not uncommon for printers anywhere to
require older PDF versions. (Not sure about TUGboat's printer yet.)

    I'd sugest switching to 1.7 

So it seems.

    and add an explicit explanation about
    producing 1.5 in the documentation.

I'm not sure I can satisfy that.
At the pdfTeX level (as you know), it can be done simply with
\pdfminorversion = 5
But XeTeX and LuaTeX are more complicated. And I don't know if there is
a LaTeX-level interface outside of \DocumentMetadata.

At any rate, The answers to this old question
https://tex.stackexchange.com/questions/202300/options-controlling-pdf-output
still look correct to me, though I didn't try them.

I'm not going to explain all that on the pretest web page or in the
manual, but I suppose I could point to it. Unless there's a better
resource someone can suggest. --thanks, karl.