If you receive this error message, it likely is because the file is opened in 'Read-only' mode.
This occurs when the file is opened via Drag and Drop.
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.