PDFs are for home printing, viewing on an iPad/iPhone/tablet, or simply to archive and protect member stories.
The Standard PDF is used for reading on devices like the iPad and Kindle Fire, printing a book at home (one page per sheet, trim the edges—done!), and to create story archives. Pocket and Mini sizes are simple art projects: they're designed to be printed at home, cut, and folded into (rather adorable) small books (simple instructions are included in the PDF). They're ideal for school projects, stocking stuffers, and home-made greeting cards.
The Standard PDF is rendered at 100 dpi and Pocket and Mini are both rendered at 150 dpi. All three use Adobe's DRM to disable image extraction or copying, while the low DPI discourages professional reproduction.