Hi Jim Kahl,
Thank you for posting your question in the Microsoft Q&A forum.
According to your sharing, the situation that you get a "Can not save, may be read only" notice when trying to save an existing PDF after adding content, is likely related to the way Windows File Explorer's Preview Pane is handling PDF files. The problem occurs because the Preview Pane keeps a lock on the file while it is displayed, preventing other programs (like your PDF editor) from saving changes. When you close File Explorer, the Preview Pane releases the lock, allowing the save to complete.
To resolve this issue, you can try disable the Preview pane by open File Explorer > go to View tab > uncheck Preview Pane
Moreover, since you said you downloaded bank statement PDF, there's a possibility that the file is marked as originating from the Internet (which is common for files downloaded from the web, known as the "Mark-of-the-Web") and Windows security features might be causing the lock. You can check that by right click the PDF file > choose Properties > in General tab, check Unblock box next to the Security section > Apply > OK.
I hope the above information is helpful. You can try workaround and let us know how it went or if you need help with any steps. I'm here to support you and make sure everything is back to work.
Thank you for your understanding and cooperation. I'm looking forward to your reply.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.