(1) Why is the Signature button greyed out (disabled)?
You used FormsCentral to create fillable forms and exported to PDF. When exporting FormsCentral forms to PDF forms, FormsCentral adds "extended rights" to PDF forms so that users can fill out the PDF forms in Adobe Reader (which is a PDF viewer, not a PDF editor).
The current version (11.5.0) of Adobe Reader for Android does not allow signatures to be added to any "Reader Extended" PDF documents (including forms). We are actively investigating and trying to resolve the problem.
(2) Is there a way to "lock" a PDF form?
You can "flatten" a PDF form after the form has been filled out and signed by a customer. Once flattened, it's not possible to change filled data or signatures. When you try to email a filled PDF form (optionally with a signature), Adobe Reader for iOS will present the E-mail Document dialog with two options: "Send Original Document" and "Send Flattened Copy". You can choose the "Send Flattened Copy" option.
Unfortunately, the feature has not been supported in Adobe Reader for Android yet.
(3) What is the easiest way to send a "flattened" copy via email?
Because the issues 1 & 2 above have not been resolved, you are not able to do this in Adobe Reader for Android. However, you can do this in Adobe Reader for iOS.
First, you need to add a Submit button to your PDF form. I am not sure if you can do this in FormsCentral. But you can use Adobe Acrobat (a paid product) to add a Submit button.
When your customer taps the Submit button in the PDF form, Adobe Reader (for iOS - for now) will present thee same E-mail Document dialog. Once the "Send Flattened Copy" option is selected, Reader will launch the default Mail app and auto-populate email address(es) specified for the Submit button action.
Please let us know if you have further questions.
"The current version (11.5.0) of Adobe Reader for Android does not allow signatures to be added to any "Reader Extended" PDF documents (including forms). We are actively investigating and trying to resolve the problem."
I'm now seeing this issue on iOS 9 in Adobe Acrobat Reader version 15.3. Any update to this?
I am also seeing this issue on my iPad in Adobe Acrobat Reader. I have a fillable PDF and the signature is greyed out. Mind you, this is on a form opened from Safari. Apparently it is a "Policy protected PDF", is there a way to turn off policy protection?