To ensure that PDFs are appearing correctly in OneNote Online, please take the following steps:

1. Make sure you’re using the latest version of OneNote.
2. Check that the PDFs you’re trying to insert are current and that their content is supported by OneNote and Microsoft.
3. Make sure that the file size of the PDFs is within the limits of what OneNote and Microsoft support.
4. Check that the file extension of the PDFs is supported by OneNote and Microsoft.
5. Ensure that you are logged in with an administrator account.
6. Confirm that OneNote and Microsoft are both up-to-date.