Please implement Crypt Filter for PDFs to open RC4 (40Bit, 128Bit) and AES (128Bit,256Bit) pdf Files inside PDFViewer.
Maybe if it helps, here is a complete open source AES/RijndaelEngine in C#, which handles 128/192/256 Keys, based on Bouncy Castles: https://github.com/adamhathcock/sharpcompress/blob/master/SharpCompress/Crypto/RijndaelEngine.cs By the way, Bouncy Castle has a complete suite of C# crypto components: http://www.bouncycastle.org/csharp/ and it also has a nice license: http://www.bouncycastle.org/csharp/licence.html