Do you want to give your Android smartphone or tablet, but be afraid of your files being exposed? Below we will see how you will make sure that all your data has been deleted. 
Before you begin, keep a backup of all your data, because as you understand if your files are deleted, they left and there is no undo.
Then connect the charger to your device because the deletion process may take a long time.
From Settings> Security> Phone Encryption (or Tablet Encryption) to get started. On Android 4.4 or older devices go to Settings> Security> Screen Lock to start the process.
You should guide the process. Set a strong password access. You don't need to encrypt it card SD, because you can simply remove it and format it. The encryption process can take over an hour.
Immediately after Settings> Backup & Restore> Factory Data Restore, then tap Reset tablet.
Once this process has been completed, your data has been deleted.
Technically though, he is still where he is described as free space. But if someone tries to retrieve it, your data is encrypted.
However, if you want to delete them completely, proceed to the next step.
The best way to get rid of the encrypted data is to replace it with new data.
There are two different ways you can do this.
You can simply upload large files to your device (such as video files that are ideal due to size) until the disc is full. Then delete the files you added.
You can also use an application like iShredder that can digitally shred data.
That's it! Your data has disappeared for good.
