Is it possible to add an external pdf into a quarto book and render it into pdf, while still keeping the page numbering of the whole quarto book?
{{< include figs/examplepdf.pdf >}} does not seem to work
\includepdf[]{examplepdf.pdf} seems only to add the pdf document, but 'as is', not layouted in the quarto book's layout (e.g. page numbering).
No, this is not possible. Quarto contains tools to generate PDF, but not to parse PDFs. The latter would be required to include PDFs the way you imagine, adjusted to the style of the document.
The pdfpages package (of which you already mentioned the \includepdf command) is likely the closest you can get.