Strange. In the Mail list the attachment is showing as a PDF.
What "Mail list"?
But when I open the mail item, the PDF is displayed inline in the mail, without any option to open it separately.
If I drag a PDF into the Mail window on my Mac, instead of Attaching it, it does show inline in the Mail window on the Mac, but on the iPhone's Mail app, it shows as a file in the mail message, not inline. Selecting the file, then displays the PDF (with the "iBooks" button).>
So there is no "button in the PDF" itself.
Not if the Mail app is displaying the PDF as inline text.
It's almost like Mail thinks the PDF is an embedded image.
How are you sending the PDF?
Do you have Stanza installed? It may be forcing the PDF inline.
<http://discussions.apple.com/thread.jspa?threadID=2469728&tstart=0>