Open your PDF file. Do this:
File > Properties… > Select “Security” tab > Select “Security Method” > Choose “Password Security”
In “Password Security” window, mark this check-box:
“Restrict editing and printing of the document….”
In “Change Allowed” choose “Commenting, filling in form fileds, …”
Enter a Password and confirm that again.
Hit “OK” button.
Save your document and close it.
Now, your document is protected with password.