Although PowerPoint provides a spelling dictionary that contains a list of the most common words, you might be working on a particular project that uses uncommon words not found in everyday parlance. You could edit a custom dictionary or load ready-made third-party dictionaries within PowerPoint. However, have you thought about creating a dictionary yourself that includes a set of words for a particular project? If you need to do a legal presentation for a client once a year, you could create a separate dictionary for just this one project, and then edit it as required within a text editor such as Notepad.
Learn how to create and edit custom dictionaries in PowerPoint 365 for Windows.