PDF documents can be secured with a password or other security methods. PSPDFKit can handle protected and secured documents and write annotations and forms into it like with a non-protected document.
Acrobat defines various restrictions, sometimes form filling is allowed for a document.
Sometimes documents are password secured but have an empty password set. This might sound paradox, but is quite usual in the PDF world. In such cases, PSPDFKit will attempt to unlock such documents with an empty password before showing the password dialog, so users don't get confused. This is the same behavior as in Adobe Acrobat.