1. My final goal is to attach an Books estimate as PDF to a mailing template with variables in CRM or best case merge the mailing template with the filled variables and the books estimate into 1 PDF which i can then mail to my customers.

I would need to add the HTML or the PDF of the estimate to the mailing template dynamically. Also i need to embed the link to the estimate, similar to what you get when you email it in Books. How would i get that from Books and integrate it into my template with the API?

How would i achieve that, can i fill my template or create a PDF directly with a function?