Some PDF annotation apps store annotations in a way that Notability can't display. If a PDF's annotations don't appear in Notability, you can use these steps to make a new PDF that shows the annotations.
- Open a PDF in the built-in app named Preview.
- Click "File > Export as PDF..." from Preview's menu bar.
- Navigate to the folder where the new PDF should be saved.
- Click "Save".
- Open the new PDF in Notability.