If you'd like PDFs to open in the same tab, you'll most likely need to turn off Stage Manager.
Stage Manager is a system feature that allows you to group apps for specific tasks or projects, arrange windows, resize them, and overlap them.
You can disable Stage Manager if you want to avoid new windows opening automatically:
- Swipe down to open Control Center.
- Find Stage Manager (or Windowed Apps) and turn it off.
Once disabled, new PDF files should open in the same tab/window as before.