Choose File & Save As and select the location where you would like to save your document, click the 'tools' dropdown and select 'general options'
This will now present you with two options - you can set a password for 'file open*' or for 'file modify*' fill out the password box for the option that fulfils your requirements and press OK. The system will ask you to confirm your password in case of any typos, hit save and your document will be saved with the security you have chosen.
*File Open - meaning a password will be required to open the document
*File Modify - document can be opened and read but a password will be required to make any changes.