This is a formality. They want to see who is using their service and how often they are using it. You will be prompted to do something with the file. When you download the page the next day, just save over the old version, assuming you have read it already. You will be able to use the Acrobat reader with many pages.
Some web sites swear by it.
I have found that pages with specifically designed lessons or multiple languages rely heavily on the .pdf format. You may want to try some other sites with puzzles and games. You will likely discover links to other sites with games on the World Wide Web. Just dive in and try them.