Some documents store content (images, text, media) in a way that Notability can't display.
As a workaround, flattening the files before importing them into Notability should nudge things into order.
To Flatten: Open the PDF in "PDF Expert" and when sharing the file choose "Flattened copy" and a share screen should appear for you to send the document to Notability.
If you have a Mac, 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.