Lock fillable PDF and use Reader to unlock and edit?

I think it's important to identify what it is that you are protecting against. Often this has not been explored in detail, there's just a feeling that it isn't secure. Here are some possibilities, there are many more.

1. Accidentally overwriting a shared original form. Solution: protect the original file with system security, nothing to do with PDF.

2. Opportunistic price or order fraud (changing the prices on a quote for example). NOTHING can protect properly against this. If your company is vulnerable to this, then it's worth someone's trouble - if the PDF is too hard to edit - to type up a new fake. If a company is open to this fraud, it has a fault in procedures; it needs to be someone's job to check the numbers against records.

3. Contract fraud: changing the terms of a contract before returning it signed. The solution here does exist, digital signatures. They can be used to prove that the client changed the contract - needs expert advice to set up something legally binding. Staff may need to be trained to check signatures (every time potentially, and NOT by just looking at the marks on the page, which are easily faked).

4. User convenience: if the user doesn't need to change the form any more, why confuse them by inviting them to fill it? This can be handled by setting fields read only but beware: don't do this just because the form is saved. Form designers forget that forms often have to be saved when half filled, to return later (after lunch, a meeting, going away to look up stuff for the form, or a crisis).

5. Protecting secrets inside the scripts of the form, for example passwords or internal pricing, or protecting the work done in writing the scripts from competitors. You can't protect this, don't even try. Be sure you don't need secrets in the form, by design.

The best way is to use digital signatures. It will not prevent the file from being edited by someone who is willing enough to do so, but it will allow you to prove that the file was editing after it was signed. And the best part is you only need Reader to use them. So you can create the file with two digital signature fields, sign the first one, send to your client, who in turn can fill in the fields and then sign it herself. The second signature field can be set to lock all the fillable fields.

You can add a password, and the client can take it off if they know the password. They will however need to pay for Acrobat in order to do this; this is all part of Adobe's plan. Bear in mind that a lot of software will ignore the password and allow editing! This is not a security measure, and you CANNOT USE SECURITY MEASURES TO STOP ANY PDF FROM BEING EDITED. So if there is some kind of fraud you are trying to prevent you need a different plan.