Adding two Outlines to pages with landscape orientation changes the orientatiuon of the used page to portrait.
Code:
    Private Sub example()
        Dim pdfDoc As PdfSharp.Pdf.PdfDocument
        pdfDoc = PdfSharp.Pdf.IO.PdfReader.Open("c:\Mixed Landscape Portrait.pdf")
        Dim pdfPage As PdfSharp.Pdf.PdfPage
        pdfPage = pdfDoc.Pages.Item(0)
        pdfDoc.Outlines.Add("Bookmark on Page 1", pdfPage)
        pdfPage = pdfDoc.Pages.Item(1)
        pdfDoc.Outlines.Add("Bookmark on Page 2", pdfPage)
        pdfDoc.Save(Me.sWorkFile)
    End Sub
The file "Mixed Landscape Portrait.pdf" contains two pages, one landscape and one portrait.
After running the example code both pages are portrait.