Note, that there is a method in PDFDoc called IsModified(). That may be enough for your situation?
If not, your best choice, assuming you’re targeting Windows desktop, and you’re okay with targeting .Net 4+ framework, is to use our PDFNet .Net 4 libraries and use the PDFViewWPF class. This is the most customizable version of our viewer. See the PDFViewWPF and PDFViewWPFSimpleTest samples in the download.
If you’re not targeting Windows desktop, please let us know what your target is.