c# - How to add annotations and highlights to an encrypted PDF file?

Question

I need to write an application, which highlights some text in a PDF file and add annotations to them.

I've used the PDF Clown library and it works well with non-encrypted files in the following way:

1. I read the PDF file A.
2. Add some annotations and highlights.
3. Write out the result to PDF file B.

File A is not changed.

This works well for non-encryped files, but does not work for encrypted ones.

How can I add highlights and annotations to an encrypted file in C# or Java?