PdfDownloadInterceptor

PdfDownloadInterceptor.cs

public static class PdfDownloadInterceptor
{
    public static async Task HandleAsync(ResponseTransformContext context)
    {
        var proxyResponse = context.ProxyResponse;
        var httpContext = context.HttpContext;

        if (proxyResponse == null)
            return;

        if (proxyResponse.Content?.Headers.ContentType?.MediaType != "application/pdf")
            return;

        var originalPdf = await proxyResponse.Content.ReadAsByteArrayAsync();

        var modifiedPdf = PdfProcessor.ChangePdf(originalPdf);

        var response = httpContext.Response;

        response.Headers.Clear();

        // Force browser download
        response.ContentType = MediaTypeNames.Application.Pdf;
        response.Headers[HeaderNames.ContentDisposition] ="attachment; filename=\"document-processed.pdf\"";
        response.ContentLength = modifiedPdf.Length;

        await response.Body.WriteAsync(modifiedPdf);

        context.SuppressResponseBody = true;
    }
}

PdfProcessor.cs

using iText.IO.Font.Constants;
using iText.Kernel.Colors;
using iText.Kernel.Font;
using iText.Kernel.Geom;
using iText.Kernel.Pdf;
using iText.Kernel.Pdf.Canvas;
using iText.Layout;
using iText.Layout.Properties;

public static class PdfProcessor
{
    public static byte[] ChangePdf(byte[] originalPdf)
    {
        using var input = new MemoryStream(originalPdf);
        using var output = new MemoryStream();

        using var reader = new PdfReader(input);
        using var writer = new PdfWriter(output);
        using var pdf = new PdfDocument(reader, writer);

        var font = PdfFontFactory.CreateFont(StandardFonts.HELVETICA_BOLD);

        int totalPages = pdf.GetNumberOfPages();

        for (int i = 1; i <= totalPages; i++)
        {
            var page = pdf.GetPage(i);
            Rectangle pageSize = page.GetPageSize();

            var pdfCanvas = new PdfCanvas(
                page.NewContentStreamAfter(),
                page.GetResources(),
                pdf
            );

            using var canvas = new Canvas(pdfCanvas, pageSize);

            // Watermark
            canvas
                .SetFont(font)
                .SetFontSize(48)
                .SetFontColor(ColorConstants.LIGHT_GRAY)
                .ShowTextAligned(
                    "GERADO VIA YARP",
                    pageSize.GetWidth() / 2,
                    pageSize.GetHeight() / 2,
                    TextAlignment.CENTER
                );

            // Rodapé
            canvas
                .SetFontSize(9)
                .SetFontColor(ColorConstants.GRAY)
                .ShowTextAligned(
                    $"Página {i} de {totalPages}",
                    pageSize.GetWidth() / 2,
                    20,
                    TextAlignment.CENTER
                );
        }

        pdf.Close();
        return output.ToArray();
    }
}

Program.cs

using ProxyYarp.Services;
using Yarp.ReverseProxy.Transforms;

var builder = WebApplication.CreateBuilder(args);

builder.Services
    .AddReverseProxy()
    .LoadFromConfig(builder.Configuration.GetSection("ReverseProxy"))
    .AddTransforms(builderContext =>
    {
        builderContext.AddResponseTransform(async context =>
        {
            await PdfDownloadInterceptor.HandleAsync(context);
        });
    }); 

var app = builder.Build();

app.Use(async (ctx, next) =>
{
    Console.WriteLine($"Request: {ctx.Request.Method} {ctx.Request.Path}");
    await next();
});

app.MapReverseProxy();

app.Run();