Users may run into font size and font type changing when copying a slide content from one PowerPoint file to another. This is also true when copying content from PowerPoint Online to PowerPoint Desktop, or vice-versa. This behavior is by Microsoft's design and they have compiled some tips and tricks on retaining style and format when copying and pasting content. 

 

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Some of these formatting styles may not work when you copy from a PowerPoint on the desktop to Powerpoint in Office online. We suggest creating a new PowerPoint document on the desktop by using the text and formatting styles from another PowerPoint then upload the PowerPoint file.