Some PDF files are marked by their creators as "protected", which means no one is allowed to copy or extract their contents.
Adobe Systems Inc©, the company that created and maintains the PDF format, requires all applications that use this format to honor this special limitation. This means that PDF2XL will not be able to convert these files.
When you open a protected PDF file, PDF2XL will notify you that the file is protected and ask if you want to open it anyway.
If you choose to open it, you will be able to view the file, but not to convert it into any other format or to copy text out of it.