it all really depends on the level of protection you're
looking for.
for most people simply right clicking the file
and "ticknig" the read only attribute is enough to deter
people from alterign your content as they are not allowed
to save the document only use "save as".
if you use a font in your presentation that is outside the
usual. ie a custom font. and save your presentation
enbedding this font, your presentation will open as read
only.