"Open in cloud on" function from an attachment does not save to cloud on with google docs.
Documents do not save to desired folders or even to the uploads folder. They are just listed underneath the "paper looking" icon. These documents cannot be moved or copied. Eventually they vanish from this list. This account is only synced with google docs.
Dropbox linked accounts experience a similar result. I contacted support on this topic and was advised you must edit the file, even if it is just to add and then remove a space character in order for the file to save to the Uploads directory. The problem with this approach is there is no way to edit a PDF and therefore no way to save it within one of your linked cloud storage accounts. The current workaround would be to use another App such as Notability to upload the file to your linked cloud storage.