I'm looking for a scanner API library to be embedded in a new app I'm developing in order to give it the feature of scanning documents (in PDF or other formats) using the built-in camera of the iPhone/iPad. Is anybody aware of such a library (...available for free or cheap...of course).
So you are looking for an Optical Character Recognition (OCR) iOS SDK.
There is 2 basic type of OCR SDKs: offline and cloud based.
Here is a few options:
ABBYY ,which is a great OCR engine, they have an offline and a cloud base solution too. So far it is the best OCR engine for iOS, very good performance and very good precision, but they are not cheap. You have to contact the sales team, and provide information for about your project for a demo SDK.
Tesseract, iOS Wrapper here, which is Google's Open Source project. It is free, but it has way worse performance than ABBYY's engine. It is very flexible, and has a big community.
Also there is some more, but I don't have any experience with these:
VeryPDF Cloud
First you should let the user take a good photo of the desired document, crop and scale it for the most accurate picture, and after that submit it to the OCR engine.