Older Postscript documents available on the Internet will sometimes Distill into PDF in the reverse page order.  This is typically because they were originally printed using one of the original Apple LaserWriter print drivers. The original LaserWriters did not flip the paper before placing it in the output tray; so, the print driver would send the document to the printer in reverse order to collate properly.

ReversePages.pdf

ReversePagescpp.cpp