PDF attachments are sending as icons but showing up as the body of the email rather than at attachment once it's received in an inbox?

On Apple Mail, in the compose window, there will be an attachment icon in the toolbar. Click that, and then on its bottom toolbar, click Options and select Send Windows-Friendly Attachments. Then either select your PDF for attachment, or Cancel, and drag/drop the PDF onto your Mail compose window.

This is the only thing. you can do that may possibly change how the mail recipients' email client (probably Outlook) handles and displays your PDF attachment.

Single page attachments may display fully open in the body of the email.

Multi-page attachments should only display as icons.

I discovered this little "feature" not so long ago, and I've been using Macs for ages. 🤯

Add a blank page to the end of the document to ensure it is a multipage .pdf

Also, you may have no control over what the recipient sees when they receive your email. How their mail client handles things is not within your control.

+1 👍🏽