iPhone users can make use of the official PowerPoint app for iOS by Microsoft to turn their PPT into PDF, a more universal file format accepted by all kinds of devices. See this tutorial to convert PowerPoint to PDF on iPhone for free. It however is not effective at all. If you have many PPT files need to convert, you should choose a more specialized PDF converter other than the MS PowerPoint app. Here we recommend this PDF Converter app by Readdle which is very powerful and yet user-friendly. You can use it to convert all kinds of files on iOS devices in just several clicks.
Converting PowerPoint presentations to PDF on iPhone
Find your PPT files on the device, could it be the stock Files app or third party file managers or even cloud storage. Then select the PPT file, and tap on the Action menu at the bottom left corner to bring it up at your screen bottom. You should find the Convert to PDF option from there. Simply touch it to convert the selected PowerPoint file to PDF on your iPhone or iPad.
When the conversion complete, you will get a prompt telling you the PDF file is ready with the options to open it in the PDF converter app or stay on the current app and convert more PPT files.