Mark-Kashman, you mention "If you have a photo file in OneDrive or SharePoint, and you go into edit mode, you can now add ink inline - like how you've been able to do with PDFs."
I am seeing the option to "Edit" a JPG image in OneDrive/SharePoint to add ink inline (introduced with https://www.microsoft.com/en-ww/microsoft-365/roadmap?filters=&searchterms=88481), but don't seem to have the ability to do so for PDF documents (which you mention was already possible before).
The closest to editing PDF in SharePoint Online that I can find with a web search is https://social.technet.microsoft.com/wiki/contents/articles/53301.edit-pdf-files-from-sharepoint-online-site.aspx (and https://www.c-sharpcorner.com/article/how-to-edit-pdf-files-in-sharepoint-online/) but that requires a third party tool.
Could you please help point me to the instructions to edit PDF documents inline in SharePoint Online? This will be a huge time-saver for our teachers!