Locking specific form fields

I am creating photography contracts and I cannot figure out how to make only certain form boxes editable. I have a base pdf file that has a few forms that I would fill out before sending to my client to complete and sign, but I want to make sure that my client would not be able to change anything that I've already typed into those form boxes. I don't want to have to force every client to download adobe acrobat and create a signature profile as I can imagine many will simply use a phone or tablet to fill out their end of the contract. If I could somehow make the contract first filled and signed by me, then lock all changes, then client can fill in their portion, that would be ideal.