Print XFA file to PDF without opening it

Question

I am attempting to auto-fill/sign/print the federal i-9 form using Spire.PDF and C#. The i9 file is an XFA form and is protected and doesn't allow for signing. However, if I fill the i9 and print to PDF, then I can sign that new file.

The step I'm getting stuck on is printing the filled i9 to a PDF file without actually opening Acrobat or having direct interaction from the end-user to specify a file name. I say 'printing' because if I just save it as a PDF file it never flattens the XFA form and remains locked against signing.

So far I have automated printing of the file using this code:

Process proc = new Process();
proc.StartInfo.Verb = "PrintTo";
proc.StartInfo.FileName = filename;
proc.StartInfo.Arguments = "\"" + printername + "\"";
proc.StartInfo.UseShellExecute = true;
proc.StartInfo.CreateNoWindow = true;
proc.Start();

and I think I might be able to force use of the Microsoft Print to PDF 'printer' here, but I don't know if there's a way to specify the file name to use so that the user isn't prompted?

If I try printing using the Spire.PDF control, I am only able to get a file with the "Please wait... If this message is not eventually replaced by the proper contents of the document, your PDF viewer may not be able to display this type of document.." message as a result.

When the form is opened to print via Acrobat I get a popup of "This form contains incomplete or invalid information. Are you sure you want to print?" If I click Yes then I can successfully print to PDF and then I can sign that file.

So, I believe whatever data-checking is happening is causing the failure to print via code and I'm hoping those wiser than I might have some ideas of ways around this issue.

Thank you in advance for your help! If you just search for Federal i9 you should find the file I'm working with. I didn't see a space to attach a file here.

This is the code that I'm using to try to accomplish my task via the Spire.PDF control.

    PdfDocument doc = new PdfDocument();
    string i9path = "locationofi9file"
    string newi9path = "locationoffilledi9file"
    doc.LoadFromFile(i9path); 
    /*fill form here*/
    doc.Form.IsFlatten = true;
    doc.SaveToFile(newi9path, FileFormat.PDF);
    doc.Close();
    doc.Dispose();

         
    doc.LoadFromFile(newi9path); 
    string file = "printi9";
    if (System.IO.File.Exists(file))
        System.IO.File.Delete(file);

    string directory = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);

    PrinterSettings settings = new PrinterSettings();
    PageSettings pages = new PageSettings();

    string printername   = "Microsoft Print to PDF";
    settings.PrinterName = printername;
    settings.PrintToFile = true;
    settings.PrintFileName = Path.Combine(directory, file + ".pdf");
        
    PrintDocument printDoc = doc.PrintDocument;
    printDoc.PrinterSettings = settings;
    printDoc.Print();

    doc.Close();
    doc.Dispose();