I think it is a interesting requirement about PDF and ebook, so I quote it here,

Many computer users have thousands of digital photos (aka pictures, images) and so, they need a tool like ACDSee or IrfanView or XnView, this is, a Photo/Picture/Image quick/rapid viewer/browser/explorer Rather than manually click on each picture to launch a [...]

I just finished a bid from RentAcoder, the title is “Web service (C#) to digitally sign PDF documents”, it is a private bid, the requirement is,

Buyer needs a C# web service that will digitally sign a PDF created using ITextSharp.
The PDF will be already created and would be updated with a digital signature by [...]