Workflow problem: iOS - attach PDF in Books to reply in Mail

I often receive Word documents which require a signature. In order to add the signature I convert to the document to PDF in Books. Once I’ve signed the PDF I cannot find a way to send the document as an attachment in an email reply in mail. It is not convenient to start a new mail from Books because this breaks the email thread. Is there a way to attach the Books PDF to the reply mail?

PS I find the lack of simple answers for questions like this in the Apple support community extremely frustrating for example in trying to solve this, all I could find was a 2011 response which does not address the requirement to attach the file to a Reply email… Apple needs to formalize and tidy up its support. This is a colossal waste of time for me.