Its actually pretty easy to corrupt any type of file on any computer safely.
1) View the file in the finder.
2) Change the file extension from ".ppt" or ".pptx" to a ".txt" You're computer will say that this could make the file unusable or unusable, make sure you click the option to use at the ".txt" file type. However, in this state its somewhat easy to recover it. If you change it back to the original extension it can be rescued from this point. If you don't see the file extension, change your folder view options.
3) Open the file after changing open the file in the text editor for Macs. In Windows computers it should will open it in "Notepad." These programs are the same thing in that they are simple text editors. You should see what appears to be random text in the window. Now, have fun and begin to delete clips of the text. It shouldn't take much to make it unusable. Make sure to save it when you're done editing it!
4) Go back to the file viewed in the finder and change the file extension back as the original file type and make sure you click the option to use it as the ".ppt" or ".pptx" file type again. The computer should recognize it as a Power Point file however, when you try to open it the file should not load. If it still opens successfully, repeat the process as needed.
Note: This can work for nearly any file type! Its actually quite fun to do this with ".jpeg"s because, it sometimes makes them look all cool and glitchy.