For API documentation, see http://cdn01.foxitsoftware.com/pub/foxit/manual/enu/FoxitPDF_SDK20_Guide.pdf
You can search the source code through https://code.google.com/p/chromium/codesearch#search/&q=file:third_party/pdfium&sq=package:chromium&type=cs
A sample consumer of this library is Chromium's PDF plugin. You can see the source for it at http://src.chromium.org/viewvc/chrome/trunk/src/pdf/