C# を使って PDF を回転させる

    // The path to the documents directory.
    string dataDir = RunExamples.GetDataDir_AsposePdf_Pages();

    Document doc = new Document(dataDir + "input.pdf");
    foreach (Page page in doc.Pages)
    {
        
        Aspose.Pdf.Rectangle r = page.MediaBox;
        double newHeight = r.Width;
        double newWidth = r.Height;
        double newLLX = r.LLX;
        //  We must to move page upper in order to compensate changing page size
        // (lower edge of the page is 0,0 and information is usually placed from the
        //  Top of the page. That's why we move lover edge upper on difference between
        //  Old and new height.
        double newLLY = r.LLY + (r.Height - newHeight);
        page.MediaBox = new Aspose.Pdf.Rectangle(newLLX, newLLY, newLLX + newWidth, newLLY + newHeight);
        // Sometimes we also need to set CropBox (if it was set in original file)
        page.CropBox = new Aspose.Pdf.Rectangle(newLLX, newLLY, newLLX + newWidth, newLLY + newHeight);

        // Setting Rotation angle of page
        page.Rotate = Rotation.on90;
    }

    dataDir = dataDir + "ChangeOrientation_out.pdf";
    // Save output file
    doc.Save(dataDir);