If you receive this error message, it likely is because the file is opened in 'Read-only' mode.
This usually occurs when the file is opened in Drawboard PDF via a Drag and Drop action.
To ensure changes are saved, click Save As... and overwrite the same file.
To ensure this does not happen further, either:
- Open PDF files directly from within Drawboard PDF
- Double-clicking PDF files when Drawboard PDF is your default PDF app.
Additionally this guide below shows you how to open PDFs properly within Drawboard PDF.