Page labels

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Viewers show the labels of the pages instead of their numbers. PageLabels holds ranges of pages, each with a numbering style, a prefix and the number of its first page:

// i, ii, iii for the front matter, then 1, 2, 3... and A-1, A-2 for the appendix
document.PageLabels.Set(0, new PdfPageLabel(PdfPageLabelStyle.LowercaseRoman));
document.PageLabels.Set(3, new PdfPageLabel(PdfPageLabelStyle.Decimal));
document.PageLabels.Set(6, new PdfPageLabel(PdfPageLabelStyle.Decimal, prefix: "A-"));

A range starts at the page given to Set and ends before the next range. Remove and Clear remove ranges.
PdfPageLabelStyle is decimal, uppercase or lowercase Roman numerals, uppercase or lowercase letters (A to Z, then AA to ZZ...), or none: the label is then the prefix alone.
PdfPage.Label and GetLabel give the label of a page; it is null when the document has no labels.
The labels are written to the document at once, as one node of a number tree; the labels of a file are read from its whole tree.