Sites like Scribd can be legal because it’s legal to have a website where people submit material, even if some of that material will be copyrighted, as long as they abide by the DMCA Safe Harbor requirements, which are that they promptly remove or block access to copyrighted material when they are provided with a DMCA violation notice from the rights holders.
I don’t know if Scribd actually complies with the DMCA; could be they just haven’t gotten sued by someone with sufficiently big pockets to take them out of business yet. But in principle they could be just fine.