Hi,
When you have the form open in LC Designer, go to File / Form Properties...
The PDF Security tab allows you to tick to require the user to input a password to open the form. If you tick this then the next time you save the file you will be prompted to set the password.When the users try to open the form in Acrobat they will have to input the correct password.
Note the second option sets a password to open the form in LC Designer. If you do not set this password as well, then a user can bypass your password by opening the form in LC, deselecting the password required and clearing the passwords. Have a read of the help file for further info.
Good luck,
Niall