You might be interested in downloading a portable version because you can’t install programs on the PCs you usually use or because you haven’t got enough space, amongst other reasons. PowerPoint might not be what you’re looking for in that case but there are other additional solutions that you should be interested in such as resorting to the web version of Microsoft Office that also includes a cloud version of PowerPoint (provided that you’ve got an Internet connection), using other office suites with slideshow and presentation tools that run on the cloud, getting hold of the mobile version of the app that Microsoft has been developing for quite some time and that can get you out of a tight spot despite its limitations or directly download other office suites that do come along with a portable version as is the case of LibreOffice or OpenOffice, two free tools that have always had our needs in mind.
We’re sorry to disappoint you but a dose of realism can be positive. LibreOffice Portable and OpenOffice Portable are the best and most straightforward options in this case. Other than that, you’ll have to take a look at a few pirate sites but we can’t guarantee that you’re going to find what you’re looking for without taking a few risks.