The simplest way to limit access by unauthorised people to your documentation is to apply a password. You may however forget your password and then encounter problems accessing your data. See Keeping Passwords / PIN Numbers Confidential.
Opening a document or spreadsheet may be impossible where the password has been forgotten or the owner is no longer available.
There are 2 Key Actions providing safeguards against this Information Security Issue. These are available in the registered version of RUSecure.