Be warned that saving as a PPS file provides little or no protection.
The little protection it does offer applies to hiding slides as well,
but that protection is very little. Anyone can open PowerPoint and then
use File > Open or Office Button > Open (in 2007) to open a PPS or PPSX
file in Normal/Edit view and make any changes they want.
One alternative that might help is to save the slides JPGs and import
the JPGs into PowerPoint. This makes it much more difficult to edit the
slides themselves but does allow presenters to hide the slides they
don't want.
--David
--
David M. Marcovitz
Author of _Powerful PowerPoint for Educators_
http://www.PowerfulPowerPoint.com/
Microsoft PowerPoint MVP
Associate Professor, Loyola University Maryland