[mindhacker]

Looks like they started with something like http://www.swftools.org/ and developed a custom viewer for the swf files.

For doc and other formats, one can use openoffice commandline tools to convert them all to pdf.

edit: Some history - http://news.cnet.com/8301-17939_109-9874527-2.html Scribd initially used the now discontinued flash paper.