If you are looking for an SDK to modify PDFs in any way, chances are you'll find those capabilities in the Adobe PDF Library SDK. The Adobe PDF Library is technology developed by Adobe Systems and distributed by Datalogics (in addition we've added things such as .NET and Java language interfaces, and have ported the Library to several other platforms). It's also the same technology used in Reader and Acrobat Pro.
There are over 800 API calls in the SDK. If you have some sort of PDF manipulation requirement - be it editing text, processing images, adding, deleting or navigating bookmarks, annotations, etc. - odds are there's an API for that in this SDK.
Learn more about the Adobe PDF Library.