Open the file you want to password protect
Open the icon tool bar and click on "file, save as."
You will now have a pop-up box entitled "save as," asking which directory and file name you want to give the file. On the tool bar in the pop-up box next to the directory drop down box are icons. Click on the "tools" drop down box.
Now click on "general options" which is where you will make the document password protected. You enter your chosen password twice and click on the box indicating "read-only recommended," then save and be sure your original document is saved in the correct directory and with the right name
Close everything and reopen the document. You will get a pop-up box asking for the password or offering a "read-only" choice. Since you've got the password, you are the only one who can modify, everyone else will be directed to read-only.