How to Rotate PDF using C#. For more details: https://kb.conholdate.com/total/net/how-to-rotate-pdf-using-csharp/

how-to-rotate-pdf-using-csharp.cs

using Aspose.Pdf;

namespace AsposeProjects
{
    class Program
    {
        static void Main(string[] args)
        {
            // Initialize license
            License lic = new License();
            lic.SetLicense("Aspose.Total.lic");

            // Load the PDF
            Document originalDoc = new Document("input.pdf");

            // Parse the pages
            foreach (Page pg in originalDoc.Pages)
            {
                // Change the page orientation
                Rectangle r = pg.MediaBox;
                double updatedHeight = r.Width;
                double updatedWidth = r.Height;
                double updatedLLX = r.LLX;
                double updatedLLY = r.LLY + (r.Height - updatedHeight);
                pg.MediaBox = new Rectangle(updatedLLX, updatedLLY, updatedLLX + updatedWidth, updatedLLY + updatedHeight);
                pg.CropBox = new Rectangle(updatedLLX, updatedLLY, updatedLLX + updatedWidth, updatedLLY + updatedHeight);

                // Rotate the page contents
                pg.Rotate = Rotation.on270;
            }

            // Save the rotated PDF file
            originalDoc.Save("rotated.pdf");

            System.Console.WriteLine("Done");
        }        
    }
}