An error occurred while saving the commentClark, Robert commented
The issue we are seeing in this case is a limitation of the UIDocumentInteractionConroller from iOS that Outlook for iOS is using to preview the PDF attachments. This previewer does not support additional ‘markup’ of a PDF file (like text boxes that are added after a PDF is saved).
The only workarounds I have been able to find are to save that attachment either locally on the device or to a cloud storage location and then utilize a separate application to review the PDF that has additional markup. OneDrive for example supports editing a PDF and therefore also supports displaying that markup. Other PDF viewers (third party products) also can be used to display that markup from what I found in my testing.
Although the workaround is there, I still feel there should be parity in the MS products, so Outlook should be using the same PDF viewer as OneDrive.