(You can actually set up two passwords: one for opening the document, and another for changing it. You can probably imagine a use for this.)
The way you put a password on a Microsoft Word document is through the Save As… box. You can do it when you first save the document or you can do it later with a visit to the Save As… box via File/Save As….
Here’s the Save As… box. Everyone’s seen it, but almost no one’s clicked the “Options” button. They should. You should. Get a Word document going and try it yourself.
When you click the Options button you get this box:
Click at the top, where it says “Show All,” then click the Security button (in the bottom row).
Now you get to set the passwords. You can use the same one for opening as for modifying, or you can use different ones for each. It is up to you. Fill in the boxes and click OK. (You don’t have to click “Protect Document…”– that’s something different, but worth looking at someday.)
When you type the passwords you’ll only see dots. So, when you click OK, Word asks you to type the password(s) to make sure you didn’t make a typo. Provide the passwords and you are all set. From now on, that document can’t be opened or modified without a password. Close the document and see for yourself. When you double-click the document you’ll be asked for a password, and if you don’t know it the document won’t open.
Nothing to it, when you know how.