Ponder will work on any web site or pdf that the individual has access to. That includes anything out on the web, it includes pay sites that you have a subscription for, it includes documents in your library that you can access, or documents and pages on your learning management system (LMS) that you share with one another.
Ponder has a list of over 300 web resources that teachers and professors have contributed, from major news sources around the world to niche academic journals, organized by subject area. You can access the list by selecting "reading list" from the menu under the yellow cog at the top right of the Ponder site.
If you don't have a convenient place to store documents you want your students to read, we also have support for handling documents stored on Dropbox or Google Drive. Put your files up there, and then send out links to your students to read and Ponder.