If someone created a PDF of that article and attached it to an email and sent it to a US Government information system, by classification guide standards it is

If someone created a PDF of that article and attached it to an email and sent it to a US Government information system, by classification guide standards it is now a classified email. In fact, this comment would be considered classified, as would yours. The article you linked to seems to indicate that the emails were considered classified based on the assertions I just made. The sentence "The United States of America has conducted drone strikes in Pakistan" was, and likely still is, top secret information protected by a special access program and email containing any sentence conveying that information in any form that is not properly marked and handled is a violation of security procedures. It is impossible to know the context of what they are investigating without seeing the emails, and neither you nor I have seen them.