- Actually register/unregister the `WorkerTask`s, used when saving each page, correctly.
To prevent issues when terminating the Worker, we purposely wait for all running `WorkerTask`s to complete first. Hence we need to actually handle `WorkerTask`s the same way in "SaveDocument" as in the rest of this file, see e.g. "GetOperatorList" and "GetTextContent".
- Access `PDFDocument` properties in a generally safe/consistent way.
While the current code works fine, given how the PDF document is being loaded, it still seems like a very good idea to be *consistent* in how we access these kind of properties (since in general you need to avoid `MissingDataException` everywhere in this file).
- Change a variable name, since there's essentially no precedent in the code-base for *local* variable names to start with an underscore.