Step - 1:
Click on file menu on the Top most tab and then click on save as.
Step - 2:
Click on Tools Tab there and select Security Options as show in below Picture.
Step -3:
Now an wizard will open as shown below there you need to enter two passwords one to secure your file from being modified and other to secure your file so that person who have the password only can read your document.
Note: Password cannot be more then 15 characters and also it should be noted that password is case sensitive.
Step -4:
After you have entered the password an new wizard will open For the confirmation of the password.
So hope these will be help full for you all a lot.
Do share your comments below about these tweak.
You can also request for any tweak if you want below.
