Every post on this topic that I've read, and admittedly, there are a lot, has comments like "why would you edit a signed form?!?! what kind of dishonest monster are you?" So forgive me for adding yet another quation to the pile.
When I have a form, with real fields that can be filled out in the form (the blue fields), I am a human who might make a mistake. I would like to save a copy of that form, still editable, for all eternity. Yes, even after I sign it, I want an unsigned, editable version of the form to remain.
Why would I want to do something so absurd, you may wonder? Well, I am human and make mistakes on the forms that I fill out. Typos. Or information changes. There are lots of legit reasons to save an editable copy of an unsigned form.
Using the signing tool, there is the toggle to "save a copy" which does NOT save a copy. It simply overwrites the fillable form with the final signed version. This is maddening.
At some point, I may need to revise the entries on that form, and then sign and date it again. Thanks to this clever not-a-copy overwrite, I need to download a new version of the form and fill it in completely from scratch, meaning I will likely make ANOTHER human typo and won't catch that until I have already signed it and stupidly trusted adobe, again, to save a copy.
It should really be asking what I want to name my new, signed file.
Security digital signatures and esignatures
The term "signing" can be confusing. The only "signing" that locks a document is applying a digital certificate to the PDF. This is different from an ink signature. The "Sign Now" and "Sign Yourself" tools apply an ink-like signature to the PDF without locking it. So you can still make changes. However, anyone could also remove the signature or change the form. The point of a digital certificate is to prevent tampering with a completed document.