1. SDK not copy whole PDF file into memory.or storage. Document.SetCache used for some inserted objects like images, or imported page resources.
2. when saving, it save new objects(new annotation or others object) to storage.or PDFStream
3. Document.Create() and then Document.SetCache can create a new PDF file.
u can use PDFMemStream and Document.OpenStream to open a pdf file in memory, it can be saved
after saved, u can get back bytes array(PDFMemStream.get_data) and length(PDFMemStream.get_size) as result.