Can ImageMagick edit PDF

You can add a watermark to a PDF using the AnnBatesStamp class. Here is some code and documentation that demonstrates this.

static void AddWatermarktoFile() 
{ 
   string _sourceFile = @"C:\LEADTOOLS22\Resources\Images\input.pdf"; 
 
   //Create AnnBatesStampComposer instance and add the created Bates stamp to it 
   using (AnnBatesStampComposer batesStampComposer = new AnnBatesStampComposer()) 
   { 
      //Set the rendering engine 
      AnnBatesStampComposer.RenderingEngine = new AnnWinFormsRenderingEngine(); 
      //Create Bates stamp container, set its size and mapper and attach it to the composer 
      AnnContainer batesStampContainer = new AnnContainer(); 
 
      //Create AnnBatesStamp and set its properties 
      AnnBatesStamp batesStamp = new AnnBatesStamp(); 
      batesStamp.Font = new AnnFont("Arial", 12); 
      batesStamp.Foreground = AnnSolidColorBrush.Create("Red"); 
      batesStamp.HorizontalAlignment = AnnHorizontalAlignment.Center; 
      batesStamp.VerticalAlignment = AnnVerticalAlignment.Center; 
      batesStamp.Logo.Angle = 45; 
      batesStamp.Logo.Font = new AnnFont("Arial", 32); 
      batesStamp.Logo.Opacity = 0.25; 
      batesStamp.Logo.StretchLogo = true; 
      batesStamp.Logo.Text = "Watermark"; 
 
      using (RasterCodecs _codecs = new RasterCodecs()) 
      { 
         using (CodecsImageInfo info = _codecs.GetInformation(_sourceFile, true)) 
         { 
            batesStampContainer.Mapper.MapResolutions(info.XResolution, info.YResolution, info.XResolution, info.YResolution); 
            batesStampContainer.Size = batesStampContainer.Mapper.SizeToContainerCoordinates(LeadSizeD.Create(info.Width, info.Height)); 
 
            batesStampComposer.Stamps.Add(batesStamp); 
 
            //Apply Bates Stamp to our container 
            batesStampComposer.TargetContainers.Add(batesStampContainer); 
 
            int pages = _codecs.GetTotalPages(_sourceFile); 
 
            for (int i = 1; i <= pages; i++) 
            { 
               using (RasterImage _image = _codecs.Load(_sourceFile, 24, CodecsLoadByteOrder.BgrOrGray, i, i)) 
               { 
                  AnnBatesStampComposer.RenderingEngine.RenderOnImage(batesStampContainer, _image); 
 
                  _codecs.Save(_image, @"C:\LEADTOOLS22\Resources\Images\watermarkedImage.pdf", RasterImageFormat.RasPdf, _image.BitsPerPixel, 1,-1,1,CodecsSavePageMode.Append); 
               } 
            } 
         } 
      } 
   } 
} 

https://www.leadtools.com/help/sdk/dh/abs/annbatesstamp.html
https://www.leadtools.com/help/sdk/tutorials/dotnet-console-add-watermark-to-multipage-file.html

@Leon D I don't think this is free. Thank you