Scribd is one of YC's flagship companies, and rightfully so I think. People forget how painful PDF viewing used to be: the viewers were slow to load, crashed browsers a lot, and spread viruses. I remember how some forums considered linking to a pdf without adding something like [pdf] after it to be a form of abuse (more technically sophisticated forums just added that on their own).

Scribd, and things like google viewer that followed, helped get us through a rough patch until browsers started implementing better pdf viewers on their own.