Hi,
By default, groff uses Times Roman, as I'd expect.
$ groff <<<'hello world!' | grep Times
%%DocumentNeededResources: font Times-Roman
%%IncludeResource: font Times-Roman
/Times-Roman@0 ENC0/Times-Roman RE
/F0 10/Times-Roman@0 SF(hello w)72 12 Q(orld!)-.1 E 0 Cg EP
$
But on most of our systems, that font's unavailable and another is used
in its place.
$ fc-match Times-Roman
n021003l.pfb: "Nimbus Roman No9 L" "Regular"
$
https://en.wikipedia.org/wiki/Nimbus_Roman_No._9_L says
Although the characters are not exactly the same, Nimbus Roman No.
9 L has metrics almost identical to Times
The "almost" niggles. :-)
Running GhostScript's ps2pdf(1) embeds the Nimbus substitute, so even
those with Times won't see it. (Unless the embedded font is a fallback;
I haven't examined the PDF enough to see; `qpdf -qdf'.)
Should I be telling groff that I want to use those Nimbus fonts so it
uses their metrics rather than the "almost identical" Times' ones?
Cheers, Ralph.