how do i attach a pdf file without it opening and pasting to the body of the message?

A single-page .pdf file will automatically display in place when attached to an email.

If you control-click (or right-click) the displayed file and select "View as Icon" it will be reduced and you can then easily drag/drop it into a preferred location in your Mail text if necessary.

Multi-page .pdf files do not display in this manner.

You're welcome.

One addition note... when you click the paper clip icon to attach a file, that file will be inserted into your mail message at the location of the cursor. Position your cursor accordingly before attaching a file.

Have you tried just dragging & dropping the PDF in the body of the compose window?