I believe it used to be quite useful when your only option to read PDFs was Acrobat which was slow and would often lock up your browser while the plugin loaded.
Now that both Firefox and Chrome include built in PDF readers and we've also got other options like SumatraPDF, I see little value in Scribd any more.
The software used to put an additional link to Scribd after each pdf link, but we stopped that a long time ago. I assume that's what you're referring to, because there was never (and isn't) anything else along those lines that I know of.
Some years ago, while helping a relative out, I saw this first hand with a bunch of Joomla documentation that had been removed to behind its paywall.
(Not that I hadn't already reached this opinion, on a more general basis.)
P.S. I would welcome a non-Scribd link to the letter. I've reached the point where I refuse to run/use Scribd. The monetization is one thing; the horrible UI and my lack of trust in it is another and is the basis for this decision.
As others said, I already have a PDF reader, and unsurprisingly, it works better than Scribd.