This interface is used to open PDF files by PDFViewerBean when the open file icon (folder) on the toolbar is clicked
by the user. The default implementation of the interface, by the PDFViewerBean objects, shows a file chooser dialog
to let the user choose a file from local or network drives.
The host application or applet can implement this interface and override the opener in PDFViewerBean to provide a custom
document opener to its users. For instance, the PDF documents might come from a database, so a custom opener might
get a list of documents from the database, let the user choose one, and then retrieve the document from the database
and feed it to the PDFViewerBean.
When the host application gets the PDF document, it can load it into the PDFViewerBean by calling one of its loadPDF methods,
which can take a file name, an input stream or a URL.