PDF documents can be protected from unauthorized access, restricting access to specific users via password.
However, any PDF application that processes or displays a PDF document must be able to read and decrypt the contents of the pages. Technically, it cannot display an encrypted text or image without first decrypting it. Therefore, a PDF application program has full access to any PDF document it can decrypt and display.
Encryption is not allowed by the PDF/A ISO standards. For that reason, encryption options should not be used when processing PDF/A documents. Otherwise, PDF/A conformance is removed from the output document.
For more information, see PDF/A.
The encryption process applies encryption to all streams (e.g. images) and strings, but not to other items in the PDF document. This means the structure of the PDF document is accessible, but the content of its pages is encrypted. The Pdf Tools SDK uses security handlers for encryption in PDF documents.
Learn how to encrypt a PDF document with the Pdf Tools SDK.