Print preferences

The catalog's /ViewerPreferences dictionary (ISO 32000-2 12.2, Table 147) tells a viewer how to open the file and what its print dialog starts with. OnePDF reads and writes it; it never enforces it. Viewers treat every entry as a hint.

Happy path

using var doc = PdfDocument.Open("in.pdf", new PdfOpenOptions { AllowEditing = true });
doc.SetPrintPreferences(PdfPrintScaling.None, PdfDuplex.DuplexFlipLongEdge, copies: 2);
doc.Save("out.pdf");                 // or doc.SaveIncremental(stream)

For a new document, pass the same values through the writer:

var options = new PdfDocumentWriterOptions
{
    ViewerPreferences = PdfViewerPreferences.ForPrint(PdfPrintScaling.None, PdfDuplex.Simplex, copies: 1),
};

The typed model

PdfViewerPreferences has one nullable property per entry. Null means absent: it is not written, and when applied to an existing document it leaves the existing entry alone.

• Page ranges are one-based and inclusive, exactly as stored: Table 147 numbers the first page 1 and stores the ranges as a flat array of integer pairs ([1 1 3 3] is page 1 and page 3). This differs from the zero-based doc.Pages index on purpose, so a file dump matches what you wrote. Ranges must be ascending and must not overlap.
• PrintArea, PrintClip, ViewArea and ViewClip are deprecated in PDF 2.0. They are still read and written, and most viewers ignore them.
• doc.ViewerPreferences reads the dictionary (null when the catalog has none); .Dictionary on the result is the raw dictionary with every entry, including ones not modelled here. Editing never drops entries it doesn't know (/Direction, /Enforce, extensions).
• PdfDocumentWriterOptions.DisplayDocumentTitle still works; an explicit ViewerPreferences.DisplayDocTitle wins over it. The PDF/UA preparer sets DisplayDocTitle in the same dictionary and leaves the print entries alone.

Errors and leniency

• Writing (SetViewerPreferences, SetPrintPreferences, the writer option): NumCopies below 1, an empty, reversed, overlapping, descending or out-of-range page range (beyond the page count) or an undefined enum value throws ArgumentException, and nothing is changed. The writer reports it from Finish, where the page count is known.
• Reading: a value of the wrong type, an unknown name or a malformed range is left null and recorded as a warning in doc.Diagnostics; nothing is thrown (under PdfOpenOptions.Strict a warning is an error, as everywhere).
• Editing needs PdfOpenOptions.AllowEditing; otherwise the setters throw InvalidOperationException.