var doc = new PdfDocument();

// Enable cross-reference streams (PDF 1.5+)
// Replaces the ASCII cross-reference table with
// a compressed binary stream — smaller files.
doc.UseCrossReferenceStreams = true;

// Enable object streams (PDF 1.5+)
// Packs multiple small objects into a single
// compressed stream — further size reduction.
doc.UseObjectStreams = true;

// Build the document as usual...
doc.Info.Title = "Compact PDF";
doc.Language = "en-US";
doc.DisplayDocTitle = true;
var root = doc.EnableTaggedPdf();

var h1 = root.AddChild(StructureType.H1);
var page = doc.AddPage();
page.AddTaggedText(h1, "Compact Serialization",
    72, 720, new PdfTextOptions {
        Font = StandardFont.HelveticaBold,
        FontSize = 20
    });

var p = root.AddChild(StructureType.P);
page.AddTaggedText(p,
    "This PDF uses cross-reference streams " +
    "and object streams for smaller file size.",
    72, 690);

doc.Save("compact.pdf");
// File size is 20-40% smaller than default!