Spire.PDF vs IronPDF: Technical Comparison Guide

Spire.PDF Image-Based Rendering

When Spire.PDF converts HTML to PDF, text is often rendered as bitmap images:

// ❌ Spire.PDF - Creates image-based PDF
PdfDocument pdf = new PdfDocument();
pdf.LoadFromHTML("<h1>Important Contract</h1>", false, true, true);
pdf.SaveToFile("contract.pdf");

// Problems with resulting PDF:
// - Text CANNOT be selected
// - Text CANNOT be searched
// - Text CANNOT be copied
// - Screen readers CANNOT read it (accessibility violation)
// - File size is MUCH larger
// - Zooming causes pixelation

// ❌ Spire.PDF - Creates image-based PDF
PdfDocument pdf = new PdfDocument();
pdf.LoadFromHTML("<h1>Important Contract</h1>", false, true, true);
pdf.SaveToFile("contract.pdf");

// Problems with resulting PDF:
// - Text CANNOT be selected
// - Text CANNOT be searched
// - Text CANNOT be copied
// - Screen readers CANNOT read it (accessibility violation)
// - File size is MUCH larger
// - Zooming causes pixelation

IronPDF True Text Rendering

IronPDF maintains text as actual text characters:

using IronPdf;

// ✅ IronPDF - Creates real text PDF
var renderer = new ChromePdfRenderer();
var pdf = renderer.RenderHtmlAsPdf("<h1>Important Contract</h1>");
pdf.SaveAs("contract.pdf");

// Result:
// ✅ Text is fully selectable
// ✅ Text is searchable with Ctrl+F
// ✅ Text can be copied to clipboard
// ✅ Screen readers work perfectly
// ✅ File size is compact
// ✅ Zooming is crystal clear

using IronPdf;

// ✅ IronPDF - Creates real text PDF
var renderer = new ChromePdfRenderer();
var pdf = renderer.RenderHtmlAsPdf("<h1>Important Contract</h1>");
pdf.SaveAs("contract.pdf");

// Result:
// ✅ Text is fully selectable
// ✅ Text is searchable with Ctrl+F
// ✅ Text can be copied to clipboard
// ✅ Screen readers work perfectly
// ✅ File size is compact
// ✅ Zooming is crystal clear

Impact on Document Usability

Spire.PDF Rendering Engine Limitations

<!-- This HTML renders incorrectly in Spire.PDF -->
<div style="display: flex; justify-content: space-between; gap: 20px;">
    <div style="flex: 1;">Column 1</div>
    <div style="flex: 1;">Column 2</div>
</div>

<div style="display: grid; grid-template-columns: repeat(3, 1fr); gap: 10px;">
    <div>Item 1</div>
    <div>Item 2</div>
    <div>Item 3</div>
</div>

<!-- CSS Variables don't work -->
<style>
:root { --primary-color: #007bff; }
h1 { color: var(--primary-color); }
</style>

<!-- This HTML renders incorrectly in Spire.PDF -->
<div style="display: flex; justify-content: space-between; gap: 20px;">
    <div style="flex: 1;">Column 1</div>
    <div style="flex: 1;">Column 2</div>
</div>

<div style="display: grid; grid-template-columns: repeat(3, 1fr); gap: 10px;">
    <div>Item 1</div>
    <div>Item 2</div>
    <div>Item 3</div>
</div>

<!-- CSS Variables don't work -->
<style>
:root { --primary-color: #007bff; }
h1 { color: var(--primary-color); }
</style>

IronPDF Modern Chromium Engine

IronPDF uses the modern Chromium rendering engine, supporting all contemporary CSS features:

using IronPdf;

// ✅ IronPDF - Uses modern Chromium rendering
var renderer = new ChromePdfRenderer();

var html = @"
<style>
    :root { --primary: #007bff; }
    .container { display: flex; gap: 20px; }
    .grid { display: grid; grid-template-columns: repeat(3, 1fr); }
</style>
<div class='container'>
    <div style='flex: 1; color: var(--primary)'>Column 1</div>
    <div style='flex: 1'>Column 2</div>
</div>
<div class='grid'>
    <div>Item 1</div>
    <div>Item 2</div>
    <div>Item 3</div>
</div>";

var pdf = renderer.RenderHtmlAsPdf(html);
// All modern CSS features render correctly!

using IronPdf;

// ✅ IronPDF - Uses modern Chromium rendering
var renderer = new ChromePdfRenderer();

var html = @"
<style>
    :root { --primary: #007bff; }
    .container { display: flex; gap: 20px; }
    .grid { display: grid; grid-template-columns: repeat(3, 1fr); }
</style>
<div class='container'>
    <div style='flex: 1; color: var(--primary)'>Column 1</div>
    <div style='flex: 1'>Column 2</div>
</div>
<div class='grid'>
    <div>Item 1</div>
    <div>Item 2</div>
    <div>Item 3</div>
</div>";

var pdf = renderer.RenderHtmlAsPdf(html);
// All modern CSS features render correctly!

Spire.PDF HTML Conversion

Spire.PDF uses the LoadFromHTML() method with multiple boolean parameters:

// NuGet: Install-Package Spire.PDF
using Spire.Pdf;
using Spire.Pdf.Graphics;
using System;

class Program
{
    static void Main()
    {
        PdfDocument pdf = new PdfDocument();
        PdfHtmlLayoutFormat htmlLayoutFormat = new PdfHtmlLayoutFormat();

        string htmlString = "<html><body><h1>Hello World</h1><p>This is a PDF from HTML.</p></body></html>";

        pdf.LoadFromHTML(htmlString, false, true, true);
        pdf.SaveToFile("output.pdf");
        pdf.Close();
    }
}

// NuGet: Install-Package Spire.PDF
using Spire.Pdf;
using Spire.Pdf.Graphics;
using System;

class Program
{
    static void Main()
    {
        PdfDocument pdf = new PdfDocument();
        PdfHtmlLayoutFormat htmlLayoutFormat = new PdfHtmlLayoutFormat();

        string htmlString = "<html><body><h1>Hello World</h1><p>This is a PDF from HTML.</p></body></html>";

        pdf.LoadFromHTML(htmlString, false, true, true);
        pdf.SaveToFile("output.pdf");
        pdf.Close();
    }
}

This approach requires:

• Creating a PdfDocument instance
• Optionally configuring PdfHtmlLayoutFormat
• Calling LoadFromHTML() with multiple boolean parameters
• Explicit Close() call after saving
• Critical: Text may be rendered as images

IronPDF HTML Conversion

IronPDF provides the ChromePdfRenderer with a clean API:

// NuGet: Install-Package IronPdf
using IronPdf;
using System;

class Program
{
    static void Main()
    {
        var renderer = new ChromePdfRenderer();

        string htmlString = "<html><body><h1>Hello World</h1><p>This is a PDF from HTML.</p></body></html>";

        var pdf = renderer.RenderHtmlAsPdf(htmlString);
        pdf.SaveAs("output.pdf");
    }
}

// NuGet: Install-Package IronPdf
using IronPdf;
using System;

class Program
{
    static void Main()
    {
        var renderer = new ChromePdfRenderer();

        string htmlString = "<html><body><h1>Hello World</h1><p>This is a PDF from HTML.</p></body></html>";

        var pdf = renderer.RenderHtmlAsPdf(htmlString);
        pdf.SaveAs("output.pdf");
    }
}

The RenderHtmlAsPdf method uses the Chromium rendering engine and produces true text output. No Close() call is required—IronPDF uses the standard dispose pattern.

Spire.PDF PDF Merging

Spire.PDF merges documents using the InsertPageRange() method:

// NuGet: Install-Package Spire.PDF
using Spire.Pdf;
using System;

class Program
{
    static void Main()
    {
        PdfDocument pdf1 = new PdfDocument();
        pdf1.LoadFromFile("document1.pdf");

        PdfDocument pdf2 = new PdfDocument();
        pdf2.LoadFromFile("document2.pdf");

        pdf1.InsertPageRange(pdf2, 0, pdf2.Pages.Count - 1);

        pdf1.SaveToFile("merged.pdf");
        pdf1.Close();
        pdf2.Close();
    }
}

// NuGet: Install-Package Spire.PDF
using Spire.Pdf;
using System;

class Program
{
    static void Main()
    {
        PdfDocument pdf1 = new PdfDocument();
        pdf1.LoadFromFile("document1.pdf");

        PdfDocument pdf2 = new PdfDocument();
        pdf2.LoadFromFile("document2.pdf");

        pdf1.InsertPageRange(pdf2, 0, pdf2.Pages.Count - 1);

        pdf1.SaveToFile("merged.pdf");
        pdf1.Close();
        pdf2.Close();
    }
}

This approach requires:

• Loading each document separately
• Using page range calculation (pdf2.Pages.Count - 1)
• Inserting into the first document (modifying it)
• Closing both documents explicitly

IronPDF PDF Merging

IronPDF provides a static Merge() method:

// NuGet: Install-Package IronPdf
using IronPdf;
using System;

class Program
{
    static void Main()
    {
        var pdf1 = PdfDocument.FromFile("document1.pdf");
        var pdf2 = PdfDocument.FromFile("document2.pdf");

        var merged = PdfDocument.Merge(pdf1, pdf2);

        merged.SaveAs("merged.pdf");
    }
}

// NuGet: Install-Package IronPdf
using IronPdf;
using System;

class Program
{
    static void Main()
    {
        var pdf1 = PdfDocument.FromFile("document1.pdf");
        var pdf2 = PdfDocument.FromFile("document2.pdf");

        var merged = PdfDocument.Merge(pdf1, pdf2);

        merged.SaveAs("merged.pdf");
    }
}

The PdfDocument.Merge() method creates a new merged document without modifying the originals. No explicit Close() calls are required.

Spire.PDF Text Addition

Spire.PDF uses a canvas-based drawing approach:

// NuGet: Install-Package Spire.PDF
using Spire.Pdf;
using Spire.Pdf.Graphics;
using System.Drawing;
using System;

class Program
{
    static void Main()
    {
        PdfDocument pdf = new PdfDocument();
        PdfPageBase page = pdf.Pages.Add();

        PdfFont font = new PdfFont(PdfFontFamily.Helvetica, 20);
        PdfBrush brush = new PdfSolidBrush(Color.Black);

        page.Canvas.DrawString("Hello from Spire.PDF!", font, brush, new PointF(50, 50));

        pdf.SaveToFile("output.pdf");
        pdf.Close();
    }
}

// NuGet: Install-Package Spire.PDF
using Spire.Pdf;
using Spire.Pdf.Graphics;
using System.Drawing;
using System;

class Program
{
    static void Main()
    {
        PdfDocument pdf = new PdfDocument();
        PdfPageBase page = pdf.Pages.Add();

        PdfFont font = new PdfFont(PdfFontFamily.Helvetica, 20);
        PdfBrush brush = new PdfSolidBrush(Color.Black);

        page.Canvas.DrawString("Hello from Spire.PDF!", font, brush, new PointF(50, 50));

        pdf.SaveToFile("output.pdf");
        pdf.Close();
    }
}

This approach requires:

• Creating font and brush objects
• Using canvas drawing methods
• Coordinate-based positioning with PointF
• Explicit resource management

IronPDF Text Addition

IronPDF uses a stamper-based approach:

// NuGet: Install-Package IronPdf
using IronPdf;
using IronPdf.Editing;
using System;

class Program
{
    static void Main()
    {
        var renderer = new ChromePdfRenderer();
        var pdf = renderer.RenderHtmlAsPdf("<html><body></body></html>");

        var textStamper = new TextStamper()
        {
            Text = "Hello from IronPDF!",
            FontSize = 20,
            VerticalOffset = 50,
            HorizontalOffset = 50
        };

        pdf.ApplyStamp(textStamper);
        pdf.SaveAs("output.pdf");
    }
}

// NuGet: Install-Package IronPdf
using IronPdf;
using IronPdf.Editing;
using System;

class Program
{
    static void Main()
    {
        var renderer = new ChromePdfRenderer();
        var pdf = renderer.RenderHtmlAsPdf("<html><body></body></html>");

        var textStamper = new TextStamper()
        {
            Text = "Hello from IronPDF!",
            FontSize = 20,
            VerticalOffset = 50,
            HorizontalOffset = 50
        };

        pdf.ApplyStamp(textStamper);
        pdf.SaveAs("output.pdf");
    }
}

The TextStamper class provides a declarative approach with named properties, eliminating the need for separate font and brush object creation.

Known Spire.PDF Problems

Accessibility Compliance

Image-based PDFs generated by Spire.PDF's LoadFromHTML() method create accessibility compliance issues:

• WCAG 2.1 compliance — Fails text accessibility requirements
• Section 508 compliance — Fails US Government accessibility standards
• ADA requirements — Fails Americans with Disabilities Act requirements
• Screen reader compatibility — Content cannot be read

IronPDF's true text rendering ensures full accessibility compliance, with screen readers able to read all document content.

Spire.PDF Strengths

• Part of comprehensive E-iceblue office suite
• Integration with other suite components (Word, Excel, PowerPoint)
• Suitable for legacy applications requiring cross-tool consistency
• Versatile PDF manipulation capabilities
• Freemium licensing option available

Spire.PDF Limitations

• Text rendered as images (not selectable, searchable, or accessible)
• Internet Explorer-dependent rendering engine
• Known font embedding problems
• Large deployment footprint
• Limited modern CSS support (no Flexbox, Grid, CSS Variables)
• Complex API requiring explicit resource management

IronPDF Strengths

• True text rendering (selectable, searchable, accessible)
• Modern Chromium-based engine with full CSS3 support
• Reliable font handling
• Moderate deployment footprint
• Comprehensive documentation and professional support
• Simple API without explicit Close() requirements
• Full Flexbox, CSS Grid, and CSS Variables support
• PDF manipulation capabilities (merge, split, stamps)

IronPDF Considerations

• Commercial licensing model
• Different API patterns from E-iceblue suite