Bug: Cannot mark up a PDF that has editing password applied
STEPS TO REPRODUCE THE ISSUE:
Open the sample PDF file (attached) in Adobe Acrobat on your PC. In Acrobat, select 'File' > 'Properties' and note that a password has been applied to restrict editing of the document with 'Commenting', 'Filling of form fields', and 'Signing' unrestricted.
Now, open this PDF on your iPhone/iPad using the Acrobat Reader app. You will find that even though the PDF has Commenting enabled, there is no option provided to the user to either add comments to the document or apply mark-up using the Apple Pencil.
If you open the same PDF file in another 3rd party PDF app, such as iAnnotate, you will find that commenting and markup functionality works as expected.