Hi Rose_04,
Thank you for reaching Microsoft Community.
May I know which app are you using in opening and writing a PDF? Is it through the Microsoft Edge?
When you zoom in, are you using the touchscreen of the device or the touchpad on the typecover?
Does it only happen in pdf? How about when doing other things like reading an online article in Microsoft Edge, does zooming in and out behave the same?
Which Surface Pro 9 configuration do you have? Intel or 5G?
If possible, kindly make a video where we could see the experience that you are having when zooming in and share it to us here. You may upload the video on OneDrive then just provide us here with the Shared link. See: Share files with OneDrive - Microsoft Support
To troubleshoot, kindly try first checking for the Surface drivers' health through the Surface app then run the Surface Diagnostic Toolkit. See: Fix common Surface problems using the Surface app and Surface Diagnostic Toolkit - Microsoft Support. After running the toolkit, see same issue persists.
Let me know how it goes and we may recommend other troubleshooting steps once we have received your responses to the questions above.
Kind regards,
Marrion