To ensure the authenticity of a document you have to sign the document with the certificate. I will show you how you can easily sign PDF document with the help of simple tool DigiSigner.
If any file can not be deleted, then you can use the program LockHunter, which removes the active connections of other applications to that file, which allow you to change or delete file.
Gmail predicts which emails might be of high importance to you so it marks them as important with a yellow icon. If your inbox ends up full of so called important emails, this is how you disable this feature.
Let me show you how to add a Google account to a Samsung Galaxy SIII so you can read you Gmail email, download apps from the Play Store and synchronize you Google Documents.