You can load PDF documents objects using the following static methods:
PdfDocumentFromFile(String, UInt32, String) - Loads a PDF document from a file
PdfDocumentFromFile(String, UInt32, String, String) - Loads a password protected PDF document from a file
PdfDocumentFromData(String, UInt32, Byte) - Loads a PDF document from data
PdfDocumentFromData(String, UInt32, Byte, String) - Loads a password protected PDF document from data