Hi, I have been using apple pencil to mark up PDF doucments on my ipad, which is linked to dropbox. Most of the time the blue pencil icon appears on the bottom right of the document, which allows me to then edit using apple pencil. However, I have a document at the moment where the blue pencil icon simply does not appear, which means I am unable to mark up the document using the apple pencil. This has been really frustrating. How can I get the blue pencil icon to appear?