So i solved the issue. The problem was that the sourcepdf had a cropbox - i only needed to correct my x and y position with that information:
PdfStamper stamper = null;
try
{
stamper = new PdfStamper(pdfSource, destination);
PdfImportedPage importatedPage = stamper.GetImportedPage(pdfTarget, 1);
PdfContentByte background;
for (int iPage = 1; iPage <= pdfSource.NumberOfPages; iPage++)
{
background = stamper.GetOverContent(iPage);
// here comes the important part
Rectangle cropBox = pdfSource.GetCropBox(iPage);
float xCorrected = 0 + cropBox.Left;
float yCorrected = 0 + cropBox.Bottom;
background.AddTemplate(importatedPage, xCorrected, yCorrected);
}
}
Take in mind that in case the pdf that you want to stamp on your original has also a cropbox, you need to reduce the x,y by x,y of that cropbox again.