Another solution is to make de PDF accessible, see this document : http://www.adobe.com/fr/enterprise/accessibility/pdfs/acro6_pg_ue.pdf
I don't know if Powerpoints are by themself more accessible than PDF. I think that the above document can give arguments to show that a given document conforms to the accessibilty rules. With Adobe Reader, you can verify if your document can pose problems. I checked a PDF presentation produced with the Beamer package (an LyX), and yes, there is a problem due to the fact that the structure of the document is complex. I think it's the TOC which is present on every slide.
Hope it helps. Siegfried.
