How do I lock some text fields after populating, but leave other fields available for enduser to complete

I have created the following form to be sent to clients.

I am a newbie stumbling around in the dark.

This form will be used initially by our staff who will access via Adobe Reader.

The staff member will type text that is appropriate to the particular client into the following fields:

- tnnClientName

- freeTrialExpiryDate

- tnnClientNameMainContact

- dynamic row

How are these fields locked once populated so that the enduser cannot edit.

The staff user will then distribute to the enduser client who will complete the outstanding fields in the document and returns to us via email submit button

I want this document to return to us as a PDF document.

Any help would be appreciated as I am not up on scripting and I copied, pasted and fiddled until I got the dynamic row the way I wanted it (hope it is right)