Uses handler for encrypted documents whose encryption dictionary's Filter is
Filter, or whose SubFilter it lists, replacing the default of that name
(StandardSecurityHandler for Standard). A later registration of the same name wins.
public PdfOptions UseSecurityHandler(ISecurityHandler handler)
The handler. Shared by every document and thread of the engine: it must keep no state between calls.
ISO 32000-2 §7.6.2, Table 20. The security handler extension point.