Updated Sample for vertical fill

UpdatedSample.cs

using QuestPDF.Elements;
using QuestPDF.Infrastructure;
using QuestPDF.Fluent;
using QuestPDF.Helpers;
using QuestPDF.Previewer;

Document.Create(doc =>
{
    doc.Page(page =>
    {
        page.Size(PageSizes.Letter);
        page.Margin(18);

        page.Header().PaddingBottom(10).Background(Colors.Black).AlignCenter().PaddingVertical(15).Text("HEADER")
            .FontColor(Colors.White).Bold();
        
        page.Content().Column(column =>
        {
            column.Spacing(10);
            column.Item().AlignCenter().Text(Placeholders.Label()).FontSize(18);
            column.Item().BorderBottom(1).PaddingBottom(2.5f).Text("Section 1").Bold();
            column.Item().Text(Placeholders.Paragraph());
            column.Item().Text(Placeholders.Paragraph());
            column.Item().BorderBottom(1).PaddingBottom(2.5f).Text("Section 2").Bold();
            column.Item().Text(Placeholders.Paragraph());
            column.Item().PaddingBottom(-5f).Text("Signature").Bold();
            
            column.Item().Dynamic(new FillAboveComponent(below =>
            {
                below.Column(col =>
                {
                    col.Spacing(10);
                    col.Item().BorderBottom(1).PaddingBottom(2.5f).Text("Section 3").Bold();
                    col.Item().Text(Placeholders.Paragraph());
                    col.Item().Text(Placeholders.Paragraph());
                });
            }));
        });

        page.Footer().PaddingTop(10).Background(Colors.Black).AlignCenter().PaddingVertical(15).Text("FOOTER")
            .FontColor(Colors.White).Bold();
    });
}).ShowInPreviewer();

public class FillAboveComponent : IDynamicComponent<int>
{
    private readonly Action<IContainer> _below;

    public FillAboveComponent(Action<IContainer> below)
    {
        _below = below;
    }

    public DynamicComponentComposeResult Compose(DynamicContext context)
    {
        var element = context.CreateElement(e => e.Width(context.AvailableSize.Width).Element(_below));
        var heightRemaining = context.AvailableSize.Height - element.Size.Height - 10f;
        var content = context.CreateElement(container =>
        {
            container.Column(column =>
            {
                column.Spacing(10);
                column.Item().Height(heightRemaining).Width(context.AvailableSize.Width)
                    .Border(1);
                column.Item().Element(element);
            });
        });
        
        return new DynamicComponentComposeResult
        {
            HasMoreContent = false,
            Content = content
        };
    }

    public int State { get; set; }
}