Save PowerPoint files in .pps format to open it in slideshow mode



If you are having a presentation, save your PowerPoint file as .pps file by going to:
File > Save as > PowerPoint Show (*.pps)

Now, your PowerPoint file will directly start the slideshow when you open it again for your presentation.

If you already have a saved PowerPoint file in .pptx format, then just rename it to .pps format.

Note: In case you are sharing the file with someone, its probably better to leave them in .pptx format as it might annoy others when it opens directly as a slideshow.