The Amber LIT Converter stuff is tricky, there are very many incarnations of that message floating around that subtly vary and make removal a pain. I believe this to be by design.
If every instance of the novaPDF message has the same code as the above, I suggest trying this:
<a href=.*?\s*class="calibre\d+">Create\s*PDF</a>\s*files\s*without\s*this\s*message\s*by\s*purchasing\s*novaPDF\s*printer\s*\(<a href=.*?\s*class="calibre\d+">.*?</a>\)
Did you try reading the regex tutorial from the manual