Writes the document to stream as a complete file, writing asynchronously.
public Task SaveAsync(Stream stream, PdfSaveOptions? options = null, CancellationToken cancellationToken = default(CancellationToken))
Where to write, from its current position. It is not disposed.
How to lay the file out; null for the defaults (a classic cross-reference table).
Cancels the writes.
ISO 32000-2 §7.5. Writes exactly the bytes Save(Stream, PdfSaveOptions) writes, from the same writer; only the
writes to stream are asynchronous.