A reply to a recent Tableau User Forum Idea, "Option To Place Legend Horizontally Across Bottom For Image And PDF".
This can be done, but not on the worksheet. If you move it to a dashboard and arrange the items, they will appear as positioned and formatted when exported to PDF and PNG (+ other image formats).
Download the example workbook and PDF. The image exported is shown above and below.
I agree it would be nice to have this behavior in the worksheets, but I wanted to point out how to do it today in 7,