This is a useful option in case you want someone to see and review your presentation, but you don't want to reveal all the tricks you used to build it (transitions, animations, pseudo-animations, screenshots...).
It doesn't really protect your file, since it is still possible to modify this file. How?
Well, it's an empirical way, but it works fine.
Once you have one of these files, with a .pps extension, just do the following:
1) right-click on its icon
2) choose "Rename"
3) change the last three letters (the extension) from pps to ppt
4) press "Enter"
5) click "YES" to the message: "If you change a filename extension, the file may become unusable. Are you sure you want to change it?"
Now your old .pps file has become a normal .ppt file, that you can open in normal Slide View, you can edit and then, eventually, save again in .pps format.
But don't tell it to your colleagues!
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B. Set the file properties as "Read-only"
A temporary solution, if you don't want your colleagues to play around with your presentation with the risk to inadvertently modify it, is to do as follows:
1) Find your presentation file using Windows Explorer
2) Right-click on it
3) Choose "Properties"
4) In the "General" tab, put a check mark in the box where it says "Read-only"
5) Click "Apply" and then "OK"
What will happen to your file?
Nothing special, but it will be protected from accidental modifications. If someone tries to modify it, and then saves it back again, he/she will be prompted to save it under a different name, thus not modifying your original version.
Obviously, if the person is smart enough, s/he can still open the "Properties" window and reverse your action, that is, remove the check mark from the "Read-only" box.
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C. Save the slides as .gif or .jpg and create a new presentation
In case you don't want colleagues or friends to be able to use your logos and pictures, and basically not understand what techniques you used to create your presentation, the best solution is this:
1) create your normal presentation in PowerPoint or open an existing one
2) go to "File" >> "Save As..."
3) under "File of type" choose either "GIF Graphic Interchange
Format (.gif)" or "JPEG File Interchange Format (.jpg)".
*You will choose the first one in case you have flat color graphics like simple illustrations and logotypes, the second one if you have photographs.
4) locate a destination folder
5) click "Save"
You will now get a dialog box asking you to choose whether you want to export only the current slide or the whole presentation. You need the whole presentation, so you will select "Yes"
Now you will find a new folder in the location you have selected before, and this new folder will contain all your slides saved as separate files. If you had a 20-slide presentation, you will obtain 20 separate files.
What do you do with all these new files?
Simple, you open a new presentation and insert each file to a new slide ("Insert" >> "Picture" >> "From File...")
So you will end having a new presentation where every slide has inserted one of the new graphic files.
The purpose is that, this way, these new slides won't be editable, but they will look exactly as the ones in your old presentation.
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D. save the presentation as HTML
My last suggestion is to review an old issue of MasterView (issue #1, art. 1) and save your presentation as HTML. You can send this version instead of the normal PowerPoint one, and it will be preserved from editing.
You can read this article in the original issue of MasterView.
