attachment

Just put the cursor above, in front, behind, or below the attachment.

When I insert a single-page PDF, the flashing insertion point is right after the PDF.

So, Return, type -- That will put the insertion point below the PDF

Or, arrow left, return, arrow up, type -- That will put insertion point above PDF

Or, arrow left, type, return -- that will push the PDF down, after what you typed.

Or, Just type. It will follow the end of the PDF on the same line. -- maybe not useful

Or, ctrl-click on the PDF and set it to view as icon. Then, same as above.

Click above the attached document representation, and the cursor should appear there. Then you can start typing.