Cacher is the code snippet organizer for pro developers

TextWatermark watermark = new TextWatermark("This is WaterMark on Attachment", new Font("Arial", 19));
using (PdfDocument doc = Document.Load<PdfDocument>(Utilities.MapSourceFilePath(FilePath)))
{
    foreach (PdfAttachment attachment in doc.Attachments)
    {
        // Check if the attached file is supported by GroupDocs.Watermark
        if (attachment.DocumentInfo.FileFormat != FileFormat.Undefined && !attachment.DocumentInfo.IsEncrypted)
        {
            // Load the attached document
            using (Document attachedDocument = attachment.LoadDocument())
            {
                // Add wateramrk
                attachedDocument.AddWatermark(watermark);

                // Save changes in the attached file
                attachedDocument.Save();
            }
        }
    }
    doc.Save();
}