Go to the page and click the "Change" link next to the sharing status at the top of the page.

To remove access, uncheck the box next to the name of the person you no longer wish to have access to the page. They'll no longer be able to access the page or see it in their pages list. On the other hand, you can also select more people to share the page with.

Then click the "Save my changes" button at the bottom of the screen.
Files are not included on publicly shared pages. The body, list, notes, and images are displayed, but files aren't.
No, it's only possible to share a page publicly in "read-only" format (i.e. people can view the page but not add to or edit it). If you share a page with friends, they will be able to edit it.
If you can't find answers here or in the forums, you can always email support and we'll get back to you within a few hours.