You can’t edit protected PDF files. That’s why they are protected: to prevent editing.

I have no experience with protected .pdf files, however a lot of people that use pdf’s for the first time don’t realize there is another tool to select and copy text (and mistake it for protected). If you are a more experienced user forget what I just said. If not, find the text selection button next to the hand tool and copy the text you want to a new file.