There are many ways to create a PDF. For an invoice HTML to PDF is the easiest (you do not want or need docbook, latex, XSL-FO, …).

For HTML to PDF there are in general two ways to it:

1. not using CSS3 paged media module. You just create a heex template for the invoice and run it through something like https://wkhtmltopdf.org or GitHub - bitcrowd/chromic_pdf: Convenient HTML to PDF/A rendering library for Elixir based on Chrome & Ghostscript · GitHub (this is easy but you can’t do some things like count pages, but could very well be enough for simple invoices)
2. using CSS3 paged media module. This gives CSS the power to produce high quality content for print. The “lessons” section here https://print-css.rocks/lessons gives a good overview of the features, for example

• the ability to count pages, know the amount of pages in a document
• remove content from the body and put it in the page margins (running content, like current chapter for a book)
• orphans/widows (do not have one single line of a para alone on a page)
• break-control (no not break directly after a heading)
• create TOC, create doc-entries with leaders (chaptername .......... page 7)
• repeat table-headers when a table is continued on a new page (or add a subtotal at the page-break)

I would use (2) if you expect your invoices to have multiple pages* or if you plan to create more complex documents for print. Beware, that you may have to use a paid tool, see the tools section of print-css-rocks. But for an easy document like an invoice, some free tools should be able to do it. (browsers do not support CSS3 paged media)

*) maybe there are some tricks to get the page-count without CSS3-paged-media pages()-counter, so you could use (1) for that, but I’m not familiar with this tooling.

I’ve used https://weasyprint.org/ successfully in the past, and it’s free.

There’s also typst, a new typesetting system that is open source as well and looks very promising. You could use its cli or create a NIF using rustler.