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.
For more information, you can read these articles:
- Copy & Paste Slides In PowerPoint and Keep Slide Formatting
- Copy and Paste PowerPoint 365
- Using the Format Painter in PowerPoint and Word
- Microsoft Copy and paste your slides
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.