The PDF Archive keeps a historic record of reports (i.e. records in the Report Archive) that have been formatted (i.e. assembled report data for which a layout and some formatting parameters have been applied in order to produce a formatted PDF document.
PDF archive items are connected to report archive items and there could be several records in the PDF archive for a specific report archive item. The PDF archive enables you to see exactly how a document looked when it was printed or sent electronically to someone. This can be useful since the layout might have been updated/changed since the report was formatted, so simply re-formatting the report archive item using the "same" layout might not give the expected result. PDF archive items are cleared when the corresponding report archive item is removed.