you might have to contact them to find out if they still support 1.1.
With all the component makers, commercial and open source, moving on to new frame works, it will become increasingly harder and harder to find libraries to leverage; leaving you one option and that is come up with a sln on your own instead of leveraging external componets. The best thing to do is open yourself up to wider range of tools and upgrade your project to at least a 2.0 framework.
If nothing come your way, as alternative to using pdf encyption symatics. If all you need is to be able encrypt the files at rest and decrypt them when you serve them. Take a look at encypting and decrypting the file with a RijindaelManged in the System.Security.Cryptography namespace.