Apple - How to produce an A4-format pdf of an man page?

Right now you are running groff twice, once as part of man -t, once on it‘s own on the PostScript file generated by man.

Try

groff -Tps -mandoc -c -P-pa4 /usr/share/man/man1/ls.1 >out.ps
ps2pdf out.ps man-ls.pdf
open man-ls.pdf

or directly

groff -Tps -mandoc -c -P-pa4 /usr/share/man/man1/ls.1 | 
    open -f -a /Applications/Preview.app

---

To have it work for all man pages accessible to man, a little script will help.

#!/bin/sh
page=$(man -W $1)
if [ -r "$page" ]; then
    groff -Tps -mandoc -c -P-pa4 "$page" | open -f -a Preview.app
fi

Save it as manA4, run chmod +x manA4 once, and then use it as manA4 ls.

---

In case you are working with compressed man pages (which macOS does not use by default) you need to uncompress them on the fly.

#!/bin/bash
page=$(man -W $1)
if [ -r "$page" ]; then
    if [[ "$page" =~ .*.gz$ ]]; then
        gzcat "$page" | groff -Tps -mandoc -c -P-pa4 | open -f -a Preview.app
    else
        groff -Tps -mandoc -c -P-pa4 "$page" | open -f -a Preview.app
    fi
fi

Edit /private/etc/man.conf by commenting out the line

TROFF       /usr/bin/groff -Tps -mandoc -c

then add this line

TROFF       /usr/bin/groff -Tps -mandoc -c -P-pa4

Now man -t will write ps files in A4 paper size.

man -t ls | open .......