i think you need to distinguish defaults (command-line options) and
metadata (although perhaps there's some overlap?).

this works:

% cat file.md   # I don't think you can specify the name of the defaults
file here, so I removed it; also not sure about ${.}
---
title: This is a simple Markdown file
author: Emiliano
---

# Heading

This is the content of the section.

% cat defaults.yaml
pdf-engine: wkhtmltopdf

% cat config.yaml
lang: en-US
papersize: a4
linestretch: 1.5
mainfont: "Palatino"
geometry:
  - top=2.5cm
  - left=3cm
  - right=3cm
  - bottom=2.5cm

% pandoc --defaults defaults.yaml --metadata-file config.yaml file.md -o
file.pdf
Loading pages (1/6)
Counting pages (2/6)
Resolving links (4/6)
Loading headers and footers (5/6)
Printing pages (6/6)
Done

On Fri, 10 Jun 2022 at 17:06, Emiliano <gattulli.emiliano-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:

> Hello everyone!
>
> I'm trying to simplify the YAML section of my Markdown files by writing
> most of my settings in a separate default file or metadata file. The idea
> is to write just a few lines of YAML in my Markdown file and then having a
> separate file (let's call it config.yaml) in which I can set everything
> else. Let me provide a practical example:
>
> > file.md
> ```markdown
> ---
> title: This is a simple Markdown file
> author: Emiliano
> default: ${.}/config.yaml
> ---
>
> # Heading
>
> This is the content of the section.
> ```
>
> > config.yaml
> ```yaml
> ---
> lang: en-US
> papersize: a4
> linestretch: 1.5
> mainfont: "Palatino"
> geometry:
>   - top=2.5cm
>   - left=3cm
>   - right=3cm
>   - bottom=2.5cm
> pdf-engine: xelatex
> ---
> ```
>
> This very example I provided does not work! Do you have any idea of how to
> solve this issue?
>
> Many thanks in advance!
> Emiliano
>
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