This is one of the way to protect your presentation files.
Let say you have created a presentation and you need to distribute it to others with password protected.
Follow the listed steps to achieve the above:
1. Open the PowerPoint and say File > New.
2. In PowerPoint, select Insert > Object.
3. In ‘Insert Object’ dialog, select the option ‘Create from file’.
4. Browse for your presentation file and click the ‘OK’ button.
5. Select the embedded object and Right click > Custom Animation
6. In Custom Animation, select Add Effect > Object Actions > Show.
7. Now save your current file as .pps format.
8. Protect the current document with password.
Distribute this .pps file to others.