How to send a PDF file as an attachment?

According to Add attachments to emails in Mail on Mac - Apple Support. "Click the Attachment button (pic of paperclip) in the toolbar, locate a file, select it, then click Choose File. You can also drag files from the desktop, the Finder, or an app into your message."

Srag & drop it to the body of your email.

Or use SHIFT+CMD+A keys.