How to Add an “APPROVED” Text Stamp to a PDF in C# Using PDF.co

Create a C# Console Application

Open a terminal and create a new console application:

dotnet new console -n PdfCoAddText
cd PdfCoAddText

The project includes a Program.cs file where you will add the sample code.

Configure the PDF.co API Key

Sign in to your PDF.co account and obtain your API key.

Store the key in an environment variable rather than placing it directly in the source code.

On macOS or Linux:

export PDFCO_API_KEY="YOUR_API_KEY"

In Windows PowerShell:

$env:PDFCO_API_KEY="YOUR_API_KEY"

Replace YOUR_API_KEY with your PDF.co API key.

Environment variables set this way apply to the current terminal session. Configure the variable through your deployment platform’s secret-management system when running the application in production.

Add the Complete C# Code

Replace the contents of Program.cs with the following code:

using System.Text;
using System.Text.Json;

const string endpoint = "https://api.pdf.co/v1/pdf/edit/add";

const string sourceFileUrl =
    "https://pdfco-test-files.s3.us-west-2.amazonaws.com/pdf-edit/sample.pdf";

const string destinationFile = "approved-invoice.pdf";

string? apiKey = Environment.GetEnvironmentVariable("PDFCO_API_KEY");

if (string.IsNullOrWhiteSpace(apiKey))
{
    throw new InvalidOperationException(
        "Set the PDFCO_API_KEY environment variable before running the application.");
}

var requestBody = new
{
    url = sourceFileUrl,
    name = destinationFile,
    password = "",
    annotations = new[]
    {
        new
        {
            x = 400,
            y = 600,
            text = "APPROVED",
            fontname = "Times New Roman",
            size = 24,
            color = "FF0000",
            pages = "0"
        }
    }
};

string jsonPayload = JsonSerializer.Serialize(requestBody);

using var httpClient = new HttpClient();
httpClient.DefaultRequestHeaders.Add("x-api-key", apiKey);

using var requestContent = new StringContent(
    jsonPayload,
    Encoding.UTF8,
    "application/json");

Console.WriteLine("Sending the PDF to PDF.co...");

using HttpResponseMessage response = await httpClient.PostAsync(
    endpoint,
    requestContent);

string responseBody = await response.Content.ReadAsStringAsync();

if (!response.IsSuccessStatusCode)
{
    throw new HttpRequestException(
        $"PDF.co returned HTTP {(int)response.StatusCode}: {responseBody}");
}

using JsonDocument responseJson = JsonDocument.Parse(responseBody);
JsonElement root = responseJson.RootElement;

bool hasError =
    root.TryGetProperty("error", out JsonElement errorProperty) &&
    errorProperty.GetBoolean();

if (hasError)
{
    string message =
        root.TryGetProperty("message", out JsonElement messageProperty)
            ? messageProperty.GetString() ?? "Unknown PDF.co error."
            : "Unknown PDF.co error.";

    throw new InvalidOperationException(message);
}

string resultFileUrl =
    root.GetProperty("url").GetString()
    ?? throw new InvalidOperationException(
        "PDF.co did not return an output file URL.");

Console.WriteLine("Downloading the edited PDF...");

byte[] resultFile = await httpClient.GetByteArrayAsync(resultFileUrl);
await File.WriteAllBytesAsync(destinationFile, resultFile);

Console.WriteLine(
    $"The edited PDF was saved as \"{Path.GetFullPath(destinationFile)}\".");

Understand the Request

The application sends the following information to PDF.co:

var requestBody = new
{
    url = sourceFileUrl,
    name = destinationFile,
    password = "",
    annotations = new[]
    {
        new
        {
            x = 400,
            y = 600,
            text = "APPROVED",
            fontname = "Times New Roman",
            size = 24,
            color = "FF0000",
            pages = "0"
        }
    }
};

Source PDF

The url property contains a direct URL to the existing PDF:

url = sourceFileUrl

The URL must be accessible to PDF.co. If the PDF is stored locally, upload it to PDF.co or another accessible storage service first.

Output Filename

The name property defines the generated filename:

name = destinationFile

In this example, the result is saved as:

approved-invoice.pdf

Text Annotation

The object inside annotations controls the text added to the PDF:

• x: Horizontal position measured from the left side of the page.
• y: Vertical position measured from the top of the page.
• text: Text to add.
• fontname: Font used for the annotation.
• size: Font size in points.
• color: Text color in hexadecimal RRGGBB format.
• pages: Zero-based pages on which the text will appear.

The example uses:

text = "APPROVED"

and the color:

color = "FF0000"

FF0000 represents red.

The page value:

pages = "0"

adds the text to the first page.

To place it on every page, use:

pages = "0-"

Adjust the Text Position

PDF coordinates begin at the upper-left corner.

• Increasing x moves the text to the right.
• Increasing y moves the text downward.

The example places the annotation at:

x = 400,
y = 600

Because PDF documents can have different page dimensions, adjust these values for your source document.

You can use the PDF Edit Add Helper available through PDF.co to inspect a document and determine suitable coordinates.

Run the Application

Run the project:

dotnet run

The application will:

1. Read the API key from the environment.
2. send the source PDF URL and annotation settings to PDF.co.
3. Check the API response for errors.
4. Download the edited PDF.
5. Save it in the project directory.

A successful run displays output similar to:

Sending the PDF to PDF.co...
Downloading the edited PDF...
The edited PDF was saved as "/path/to/project/approved-invoice.pdf".

Review the Output

Open approved-invoice.pdf.

The result should contain the original sample invoice with the word APPROVED added in red on the first page.

The output is not a blank PDF created from scratch. It is an edited copy of the source invoice.

Confirm that:

• The original invoice content remains visible.
• APPROVED appears on the first page.
• The text is red.
• The text uses the requested font and size.
• The text appears at the intended coordinates.