For PowerPoint users:
Go to File, then click on Export, then click on Create PDF/XPS Document and again click on Create PDF/XPS.
A dialog box will open and there you can choose where you want to save your PDF File. You can also change the name of it. Once you are done, click Publish.
For Google Slides users:
Click on File, go to Download as, choose PDF document and you are done!
This option is great if you want to send your slides presentation to someone who does not have PowerPoint nor has a Google account. It is also useful if you have chosen a corporative template and you want to have it in PDF as your company presentation to send by e-mail, or even if you want to print it to make brochures.