Make the form fields in a PDF fillable again. Some documents ship with their fields marked read-only, so the boxes are visible but refuse to accept input - usually because the form was flattened on export, or locked deliberately after being filled once. This clears that flag and leaves the fields editable.
Then the form was flattened rather than locked - the fields no longer exist as fields, and what you see is fixed page content shaped like boxes. Nothing can restore them; use Add Text to PDF to write on top instead.
No, it deals only with the read-only state of form fields. For an encrypted document use Remove PDF Password, and for permission flags use Change PDF Permissions.
Yes, values already in the fields stay. They simply become editable again.
Files are uploaded over an encrypted connection and processed on CoonTool's own server, not in your browser. The working copy is deleted automatically once the job is done, and in any case within 24 hours. Nothing is stored permanently and no account is required.