Decrypt or encrypt a PDF

using OnePdf;
 
// Encrypt: AES-256 with a password to open the file and an owner password.
using (PdfDocument doc = PdfDocument.Open("report.pdf", new PdfOpenOptions { AllowEditing = true }))
{
    doc.Save("locked.pdf", new PdfSaveOptions
    {
        Encryption = PdfEncryptionAction.Set,
        NewEncryption = new PdfEncryptionOptions
        {
            UserPassword = "open sesame",
            OwnerPassword = "owner secret",
            Permissions = PdfPermissions.Print | PdfPermissions.PrintHighQuality,   // printing only
        },
    });
}
 
// Decrypt: open with either password, then save without encryption.
try
{
    using PdfDocument doc = PdfDocument.Open("locked.pdf", new PdfOpenOptions { Password = "open sesame", AllowEditing = true });
    Console.WriteLine($"{doc.Encryption?.Cipher}, opened as {doc.AccessLevel}, permissions: {doc.Permissions}");
    doc.Save("unlocked.pdf", new PdfSaveOptions { Encryption = PdfEncryptionAction.Remove });
}
catch (PdfPasswordException)
{
    Console.WriteLine("wrong or missing password");
}
 
using (PdfDocument plain = PdfDocument.Open("unlocked.pdf"))
    Console.WriteLine($"unlocked.pdf: encrypted={plain.Encryption is not null}, {plain.PageCount} pages");
 
// Many files have only an owner password. They open without one, and the same save writes a plain copy.
using (PdfDocument doc = PdfDocument.Open("opm1644.pdf", new PdfOpenOptions { AllowEditing = true }))
{
    Console.WriteLine($"opm1644.pdf: {doc.Encryption?.Cipher}, opened as {doc.AccessLevel}");
    doc.Save("opm1644-plain.pdf", new PdfSaveOptions { Encryption = PdfEncryptionAction.Remove });
}

Aes256, opened as User, permissions: Print, PrintHighQuality
unlocked.pdf: encrypted=False, 6 pages
opm1644.pdf: Rc4, opened as User