I want my PDF to have extra usage rights for people opening the document in Adobe Reader, but I can't find how to achieve this using iText.
The technology that's used to add extra usage rights to a PDF document is based on asymmetric key cryptography. A Reader-enabled PDF contains a digital signature that needs to be validated correctly for the extra usage rights to be unlocked. The viewer of the document (in this case Adobe Reader) has the public key and the authoring software for the document (for instance: Adobe Acrobat) has the private key.
Because iText, nor any other non-Adobe software, doesn't have access to Adobe's private key, you can only enable documents using Adobe products.