Locking pdf’s for a website

I am trying to find out how to lock a pdf – My site will allow a user to download an (xml) pre-populated application. The programming is done in ‘C’ and I am trying to figure out how to lock the pdf application so the user cannot change any information that it is populated with before submitting the application online. Currently when the app is downloaded in Acrobat the fields can be manually changed.

Thank you!