Rob,
Safari will open a PDF on download automatically.
That's correct, however, Safari doesn't open the pdf in Safari. Rather, it opens the pdf in whatever pdf reader is the default on the Mac. Unless you've changed the default, this is typically Preview. That was my point.
In order for the iPad or iPhone to open a pdf, there must be a pdf application available. Such apps are Books, Acrobat, and Goodreader.