If you are having a presentation, save your PowerPoint file in .ppsx (PowerPoint Show) format by going to:
File > Save as > PowerPoint Show (*.ppsx)
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 .ppsx format.
Note: In case you are sharing the file with someone, it’s probably better to leave them in .pptx format as it might annoy others when it opens directly as a slideshow.
