Google Docs supports to upload and preview PDF, It can save PDF page as image, and I notice the default name is gview, and format is png, so I think it seems to base on GSView.
GSview is a graphical interface for Ghostscript.Ghostscript is an interpreter for the PostScript page description language used by laser printers.For documents following the Adobe PostScript Document Structuring Conventions, GSview allows selected pages to be viewed or printed.GSview requires
Ghostscript.GSview is available for Windows, OS/2 and Linux.