c# .NET CORE adding image with transparency to exi

My goal is to add company logo to every page of an existing pdf(not watermark).

Due to pdf file and logo specifics, I can only place the logo on top of the pdf content(not underneath) and the logo has to support transparency.

One more limitation is I have to use .NET Core.

Posting this with an answer, because I could not find a clear solution. Suggestions/corrections/improvements are welcome.

Hope someone finds this useful.

The newest iTextSharp library to support .NET Core is iText7 however I cannot use it legitemately; neither making my code open source, nor purchasing the licence is an option for me. Therefore I use old, third party library:

Install-Package iTextSharp.LGPLv2.Core

Latest version, the one I'm using, at the time of this post is 1.3.2

Following usings are required

using System;
using System.Drawing.Imaging;
using System.IO;
using iTextSharp.text;
using iTextSharp.text.pdf;

To acheve image transparency in pdf, image has to be opened in a correct format

var preImage = System.Drawing.Image.FromFile(imagePath);
var image = Image.GetInstance(preImage, ImageFormat.Png);

When adding the image, it is also important to not select the image to be inline

canvas.AddImage(image);//do not put .AddImage(image, true);

Here is all the code

var imagePath = "logo.png";
var pdfPath = "edit_this.pdf";

//load pdf file
var pdfBytes = File.ReadAllBytes(pdfPath);
var oldFile = new PdfReader(pdfBytes);

//load image
var preImage = System.Drawing.Image.FromFile(imagePath);
var image = Image.GetInstance(preImage, ImageFormat.Png);
preImage.Dispose();

//optional: if image is wider than the page, scale down the image to fit the page
var sizeWithRotation = oldFile.GetPageSizeWithRotation(1);
if (image.Width > sizeWithRotation.Width)
    image.ScalePercent(sizeWithRotation.Width / image.Width * 100);

//set image position in top left corner
//in pdf files, cooridinates start in the left bottom corner
image.SetAbsolutePosition(0, sizeWithRotation.Height - image.ScaledHeight);

//in production, I use MemoryStream
//I put FileStream here to test the code in console application
using (var newFileStream = new FileStream("with_logo.pdf", FileMode.Create))
{
    //setup PdfStamper
    var stamper = new PdfStamper(oldFile, newFileStream);

    //iterate through the pages in the original file
    for (var i = 1; i <= oldFile.NumberOfPages; i++)
    {
        //get canvas for current page
        var canvas = stamper.GetOverContent(i);
        //add image with pre-set position and size
        canvas.AddImage(image);
    }

    stamper.Close();
}

This code works with local files. In my (real world) case, I receive pdf files as Base64 string, add a logo from local storage, convert it back to Base64 string and output it on a web-page.

I open the image as PNG forcefully(hardcoded) because I control what extension does the logo have. If necessary you can dynamicaly set the image format.