During the presentation I need to access a text file on my computer. However, it is required that the presentation file is displayed in full screen during the presentation.
This will still have a full screen presentation on your screen, but should work anyway:
Share Powerpoint (or whatever you use - but DO NOT share your "screen") and start the full screen view. Then alt-tab to your text document when you need it. The full screen presentation will remain in your background and in your audience's zoom, but you can see your text file.
However, check with a friend first (or a second computer, on your own) to make sure it works, and to get used to the procedure.
When you ask zoom to share your screen with the other participants zoom aks you whether you want to show the whole screen or just one window. Share the window with your document (pdf, word, whatever).
Then everyone on the call can see the document. You can scroll through it for them.
You will need permission from the meeting organizer to share your screen.