Fit Multiline Text Inside a Custom Stamp Annotation in PDFs with RadPdfProcessing

Environment

Version	Product	Author
2025.3.806	RadPdfProcessing	Yoan Karamanov

Description

This article shows how to render multiple lines of text so they fit inside the fixed rectangle of a custom StampAnnotation by dynamically determining the maximum usable font size. Unlike predefined stamp names, a custom stamp appearance requires supplying a visual form (a FormSource) that draws both background styling and text content.

When using a custom name (not one of the predefined StampAnnotationPredefinedNames), it is recommended to prefix the name with # (e.g., #CustomStamp) so external PDF viewers (like Adobe Acrobat) don't overwrite the custom appearance if the stamp is moved.

Solution

To fit multiline text inside a stamp rectangle:

1. Create a StampAnnotation with a fixed Rect on a page.
2. Build a custom FormSource that will serve as the annotation's appearance (Content.NormalContentSource).
3. Measure each line by incrementally increasing the font size in a Block until the rendered dimensions exceed the available width/height and keep the maximum size that fits.
4. Use the minimum of all line-specific maximum sizes so every line fits within the rectangle.
5. Draw a decorative background (border, fill, dash pattern) and then render each line vertically stacked.
6. Export the document to PDF.

Below is a complete example demonstrating these steps.

[C#] Measure and Fit Multiline Text in a StampAnnotation

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using Telerik.Documents.Primitives;
using Telerik.Windows.Documents.Fixed.FormatProviders.Pdf;
using Telerik.Windows.Documents.Fixed.Model;
using Telerik.Windows.Documents.Fixed.Model.Annotations;
using Telerik.Windows.Documents.Fixed.Model.ColorSpaces;
using Telerik.Windows.Documents.Fixed.Model.Editing;
using Telerik.Windows.Documents.Fixed.Model.Resources;

List<string> lines = new List<string>() { "1st line", "second line", "3rd line",};
RadFixedDocument document = CreateTextAnnotation(lines);

string exportFileName = "StampAnnotation.pdf";
File.Delete(exportFileName);
using (Stream output = File.OpenWrite(exportFileName))
{
    PdfFormatProvider provider = new PdfFormatProvider();
    provider.Export(document, output, TimeSpan.FromSeconds(20));
}

Process.Start(new ProcessStartInfo
{
    FileName = exportFileName,
    UseShellExecute = true
});

RadFixedDocument CreateTextAnnotation(List<string> lines)
{
    RadFixedDocument radFixedDocument = new RadFixedDocument();
    RadFixedPage page = radFixedDocument.Pages.AddPage();

    StampAnnotation stampAnnotation = page.Annotations.AddStamp(new Rect(100, 100, 100, 100));
    stampAnnotation.Name = "#Test";

    FormSource annotationForm = CreateContentFormWithText(stampAnnotation.Rect, lines);
    stampAnnotation.Content.NormalContentSource = annotationForm;

    return radFixedDocument;
}

static FormSource CreateContentFormWithText(Rect stampRectangle, List<string> lines)
{
    FormSource textForm = new FormSource();
    textForm.Size = new Size(stampRectangle.Width, stampRectangle.Height);

    FixedContentEditor textFormEditor = new FixedContentEditor(textForm);

    using (textFormEditor.SaveProperties())
    {
        textFormEditor.GraphicProperties.IsFilled = true;
        textFormEditor.GraphicProperties.IsStroked = true;
        textFormEditor.GraphicProperties.StrokeThickness = 2;
        textFormEditor.GraphicProperties.StrokeColor = new RgbColor(92, 229, 0);
        textFormEditor.GraphicProperties.FillColor = new RgbColor(213, 222, 226);
        textFormEditor.GraphicProperties.StrokeDashArray = new double[] { 17, 4 };
        textFormEditor.DrawRectangle(new Rect(textFormEditor.Position.Matrix.OffsetX, textFormEditor.Position.Matrix.OffsetY, stampRectangle.Width, stampRectangle.Height));
    }

    double calculatedFontSize = MeasureFontSize(textForm, textFormEditor, lines);
    textFormEditor.TextProperties.FontSize = calculatedFontSize;

    double offset = 0;
    foreach (string line in lines)
    {
        textFormEditor.Position.Translate(0, offset);
        textFormEditor.DrawText(line);
        offset += calculatedFontSize;
    }

    return textForm;
}

static double MeasureFontSize(FormSource textForm, FixedContentEditor formEditor, List<string> lines)
{
    List<double> fontSizes = new List<double>();

    foreach (string line in lines)
    {
        double fontSize = 0;
        Size textSize = Size.Empty;
        Block block = new Block();
        block.TextProperties.Font = formEditor.TextProperties.Font;
        while (textSize.Width < textForm.Size.Width && textSize.Height < textForm.Size.Height)
        {
            block.TextProperties.FontSize = ++fontSize;
            block.InsertText(line);
            textSize = block.Measure();
            block.Clear();
        }

        fontSizes.Add(fontSize);
    }

    return fontSizes.Min();
}

See Also

• Stamp Annotation
• FixedContentEditor
• FormSource Overview
• PdfProcessing Overview