Hi,
As long as QuarkXPress 8 demo is not expired (Before 60 days of its installation) it will be fully functional version.
You can create PDFs from QuarkXPress 8, you can also down save to QuarkXPress 7 document from QuarkXPress 8.
You can use all the features of QuarkXPress 8, however after demo is expired (after 60 days) you will not able to save the documents and PDFs will have water marks
I hope this information helps