Docotic.Pdf library can compare PDF documents for you.
You can compare different versions of a document using one of the DocumentsAreEqual methods.
These methods compare all of the PDF objects, ignoring some properties such as trailer IDs and creator info.
All standard properties (author, creator, keywords, producer, subject, and title) can be read and changed.
Any XMP schema can be read and changed. Use Metadata class to access custom metadata properties, well-known schemas like XMP Core/XMP Basic and Dublin Core, and other schemas.