Cross reference.

commit c3428341b64521d18c73d5f3cc064e3e05ddf95a
Author: G. Branden Robinson <g.branden.robinson@gmail.com>
Date:   Wed Apr 26 04:36:05 2023 -0500

    [pdfpic]: Refactor (`sy` -> `pso`).

    * tmac/pdfpic.tmac: Migrate gathering of image dimensions from `sy` and
      a temporary file to `pso`.

      (pdfpic@cleanup): Drop `pdfpic*temporary-file` string.

      (pdfpic@get-image-dimensions): Remove redirection.  Invoke `pso`, not
      `sy`.

      (PDFPIC): Stop constructing `pdfpic*temporary-file` string.  Stop
      testing `systat` register.  Stop sourcing and deleting temporary file.

    We keep the temporary directory handling because we will need it for the
    `PSPIC` fallback logic, but it also promises to be really painful to fix
    that before we have more formatter support for string traversal.  See
    <https://savannah.gnu.org/bugs/index.php?64114>.

Affects groff 1.22.4.

$ cat peng.ms
.LP
Here is a picture of a penguin.
.PDFPIC unwritable/penguin.pdf
$ ls -dl unwritable
dr-x------ 2 branden branden 4096 Apr 26 05:42 unwritable
$ ls unwritable/
penguin.pdf
$ groff -U -ms peng.ms >| peng.ps
I/O Error: Couldn't open PostScript file 'unwritable/penguin.eps'
troff: peng.ms:3: can't open 'unwritable/penguin.eps': No such file or directory

Switching to the 'pdf' device...

$ groff -U -ms -Tpdf peng.ms >| peng.pdf
$ echo $?
0

This is because a temporary directory was correctly used for construction of the file that was sourced to read the image dimensions.  See bug #64061 for more on that process than you ever wanted to know.

The program anonymously complaining of an I/O error would appear to be pdf2ps.  From poppler-utils--try to contain your surprise.