This is solved by exporting your Pages document to PDF, opening it in Preview, and using the ⌘-key, select just those pages you want to send via email (as a PDF attachment). Then, from the file menu > Print… select the Pages option ⊙ Selected Pages in Sidebar. Next, click the PDF button at the bottom, and from its menu, choose Send in Mail. This will attach a PDF to a new Mail compose window containing only the pages you selected from the PDF in Preview. In Apple Mail, you can control-click or right-click on that PDF attachment and select Quick Look to confirm the Pages that are in it.
Since Apple Mail is not a word processing application, it won't understand, or retain formatting of what you are attempting to copy/paste from Pages, which also happens to require body text and images as separate copy/paste operations — if you were doing so to another Pages document.
The first paragraph guarantees that the recipient's Mail client does not tamper with the sent content, and that they can view your creative work exactly as you intended with a PDF reader.