Hi,
Welcome to Microsoft Community.
Let us make sure that the contents on the problem document is not a font picture.
Meanwhile let us try Opening the problem documents from the new PowerPoint document.
Open a new PowerPoint document, click on ‘File’, ‘open’ then let us navigate to the problem document and open it and check if you are able to edit the documents.
I hope the above suggestion helps. If you need further assistance, get back to us and we will be glad to assist you.
Thank you.