Gets all the embedded files in this document.
PDF documents can contain a set of named files. These files can
then be referenced by name elsewhere in the document.
The most common case in which named files are used is in the
case of PDF portfolios. Adobe now uses the name Portfolio for this
type of document but in the PDF specification they are known as
Portable Collections. While the names may vary two are the
PDF Portfolios appear as one PDF document but actually contain
many. Typically the first document represents the visible face of
the collection when opened. After the collection is open you can
switch between the embedded PDFs.
PDF portfolios can be used to represent a folder of PDFs. For
example you might find a portable collection being used to display
multiple emails within a single PDF Portfolio.