Scribd is one of those dubious mixed blessings of the internet. It was originally envisioned as a place to post non-copyrighted materials, and things that were old enough to be "out of copyright" or public domain.
But as with any download site, Scribd has become a haven for pirates, and a huge repository for illegally copied materials.
I have mixed feelings about this. I have downloaded several books from Scribed that are technically still under copyright but were 1) issued in VERY limited editions and are out of print, impossible to buy, or extremely prohibitive to buy, or 2) were never issued in the USA, and although may still perhaps be available, they are difficult, if not impossible. to find in the USA.
If something were available through easy-to-get, affordable, or public loan (library) means, then I would probably not get it from Scribd. But there are somethings that I will admit that I have no problem getting this way. And I'm a graphic designer, and pretty much live and die by the application of copyright laws...