"This file is no longer needed, and will be deleted in X days"

There is a notable difference between PDF files and iPaper flipbooks.

PDF deletion does NOT affect your existing flipbooks.

Once a PDF file has been processed into a flipbook, it is no longer needed and is basically just taking up space/cluttering up your Library.
Because of this, the system will automatically remove PDF files (which have been processed into flipbooks) after a while.
The color of the icon indicates how long until the file will be removed.

It is of course still possible for readers to click Download PDF, even though the PDF file has been removed from Library.

If you need the PDF file for anything (re-processing f.ex.) you can download it from the flipbook. See below :)