Replacing Specific Text in PDF Spanning Multiple TextFragments inTelerik PdfProcessing

Updated on Mar 31, 2026

Environment

Version	Product	Author
2026.1.210	RadPdfProcessing	Desislava Yordanova

Description

When processing a PDF using TextFragment objects in Telerik PdfProcessing, text may be split into fragments in an arbitrary manner due to the internal structure of PDF files. This behavior can lead to scenarios where text fragments do not align with the desired words or phrases, making it difficult to replace specific text. There is no direct way to edit text on a character-by-character basis within a TextFragment.

This knowledge base article demonstrates a sample approach how to replace text spanning multiple TextFragments in Telerik PdfProcessing.

Solution

To replace specific text, even if it spans multiple TextFragment objects, follow these steps:

1. Iterate through the pages of the PDF to collect all TextFragment objects.
2. Concatenate the text from the fragments and search for the desired string.
3. Identify which fragments the text spans.
4. Replace or reconstruct the fragments as needed.

Here is a code example:

csharp

static void Main(string[] args)
{
    PdfFormatProvider pdf_provider = new PdfFormatProvider();
    RadFixedDocument document;
    using (Stream stream = File.OpenRead("example.pdf"))
    {
        document = pdf_provider.Import(stream, TimeSpan.FromSeconds(10));
    }
    string textToRemove = "undersigned borrowers";
    string textToReplace = "REDACTED";

    foreach (RadFixedPage page in document.Pages)
    {
        ReplaceTextInPage(page, textToRemove, textToReplace);
    }

    string outputFilePath = "result.pdf";
    File.Delete(outputFilePath);
    using (Stream output = File.OpenWrite(outputFilePath))
    {
        pdf_provider.Export(document, output, TimeSpan.FromSeconds(10));
    }

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

private static void ReplaceTextInPage(RadFixedPage page, string textToRemove, string textToReplace)
{
    bool found = true;
    while (found)
    {
        found = false;
        List<TextFragment> textFragments = page.Content.OfType<TextFragment>().ToList();
        if (textFragments.Count == 0) break;

        // Combine text and track fragment positions
        StringBuilder sb = new StringBuilder();
        int[] fragmentStartPositions = new int[textFragments.Count];
        for (int i = 0; i < textFragments.Count; i++)
        {
            fragmentStartPositions[i] = sb.Length;
            sb.Append(textFragments[i].Text);
        }

        string fullText = sb.ToString();
        int matchPos = fullText.IndexOf(textToRemove, StringComparison.Ordinal);
        if (matchPos < 0) break;

        found = true;
        int matchEndPos = matchPos + textToRemove.Length;

        // Find affected fragments
        int firstFragIdx = -1;
        int lastFragIdx = -1;
        for (int i = 0; i < textFragments.Count; i++)
        {
            int fragStart = fragmentStartPositions[i];
            int fragEnd = fragStart + textFragments[i].Text.Length;
            if (firstFragIdx == -1 && matchPos >= fragStart && matchPos < fragEnd)
                firstFragIdx = i;
            if (matchEndPos > fragStart && matchEndPos <= fragEnd)
                lastFragIdx = i;
        }

        if (firstFragIdx == -1 || lastFragIdx == -1) break;

        // Adjust text in affected fragments
        string prefix = textFragments[firstFragIdx].Text.Substring(0, matchPos - fragmentStartPositions[firstFragIdx]);
        int matchEndInLastFrag = matchEndPos - fragmentStartPositions[lastFragIdx];
        string suffix = textFragments[lastFragIdx].Text.Substring(matchEndInLastFrag);

        if (firstFragIdx == lastFragIdx)
        {
            textFragments[firstFragIdx].Text = prefix + textToReplace + suffix;
        }
        else
        {
            textFragments[firstFragIdx].Text = prefix + textToReplace;

            if (!string.IsNullOrEmpty(suffix))
            {
                textFragments[lastFragIdx].Text = suffix;
            }
            else
            {
                page.Content.Remove(textFragments[lastFragIdx]);
            }

            for (int j = lastFragIdx - 1; j > firstFragIdx; j--)
            {
                page.Content.Remove(textFragments[j]);
            }
        }
    }
}

Notes:

• This approach handles text that spans multiple fragments by first concatenating all text and then mapping the text back to the fragments.
• Modify and extend this sample as necessary to fit the specific requirements of your PDF documents.

See Also

• TextFragment
• TextSearch