How do i prevent changes to a pdf file ?

i use adobe reader professional 9.. i used to edit a pdf form and fill the info in it..

and now i want to lock it so no one can edit this form anymore..

how do i do this ???

how can i prevent any changes to the fields in that form ??

thanksHow do i prevent changes to a pdf file ?
encrypt the file..which no one can open it without ur passord..type in file encrypter on google,..

glary utitlities works nice too

http://www.download.com/Glary-Utilities/3000-2094_4-10508531.html