using (var srcDoc = PdfDocument.Load(@"e:\10\sample.pdf"))
{
    using (var dstDoc = PdfDocument.CreateNew())
    {
        for (int i = 0; i < srcDoc.Pages.Count; i += 2)
        {
            //Add a new page to the end of the target document. 
            //Page width is the sum of the widths of the imported pages. 
            //Height is the maximum height of imported pages.
            var combinedPage = dstDoc.Pages.InsertPageAt(dstDoc.Pages.Count, srcDoc.Pages[i].Width + srcDoc.Pages[i + 1].Width, Math.Max(srcDoc.Pages[i].Height, srcDoc.Pages[i + 1].Height));
            //Import pages i and i + 1 from source document to target document at position 0 and 1, respectively.
            dstDoc.Pages.ImportPages(srcDoc, $"{i + 1},{i + 2}", 0);
            //Move page content from the imported page at position 0 to the newly created page without offset.
            MovePageContent(dstDoc.Pages[0], combinedPage, 0, 0);
            //Move page content from the imported page at position 1 to the newly created page, offset by the width of the original page. 
            MovePageContent(dstDoc.Pages[1], combinedPage, dstDoc.Pages[1].Width, 0);
            //Generate content of newly created page.
            combinedPage.GenerateContent();
            //Move page annotations
            MovePageAnnotations(dstDoc.Pages[0], combinedPage, 0, 0);
            MovePageAnnotations(dstDoc.Pages[1], combinedPage, dstDoc.Pages[1].Width, 0);
            //delete imported pages
            dstDoc.Pages.DeleteAt(1);
            dstDoc.Pages.DeleteAt(0);
        }
        dstDoc.Save(@"e:\10\test.pdf", SaveFlags.NoIncremental);
    }
}

private static void MovePageAnnotations(PdfPage from, PdfPage to, float offsetX, float offsetY)
{
    if (from.Annots == null || from.Annots.Count == 0)
        return;
    to.CreateAnnotations();
    foreach (var annot in from.Annots)
    {
        var dict = annot.Dictionary.Clone() as PdfTypeDictionary;
        if (dict.ContainsKey("P"))
            dict.Remove("P");
        var newAnnot = PdfAnnotation.Create(dict, to);
        to.Annots.Add(newAnnot);
        if (offsetX > 0.00001 || offsetX < -0.00001 || offsetY > 0.00001 || offsetY < -0.00001)
        {
            var r = newAnnot.Rectangle;
            r.left += offsetX;
            r.right += offsetX;
            r.top += offsetY;
            r.bottom += offsetY;
            newAnnot.Rectangle = r;
        }
        //foreach (var obj in newAnnot.NormalAppearance)
        //   TranslateObject(obj, offsetX, offsetY);
        //newAnnot.GenerateAppearance(AppearanceStreamModes.Normal);
    }
}

private static void MovePageContent(PdfPage from, PdfPage to, float offsetX, float offsetY)
{
    foreach (var obj in from.PageObjects)
    {
        var clonedObj = obj.Clone();
        if (offsetX > 0.00001 || offsetX < -0.00001 || offsetY > 0.00001 || offsetY < -0.00001)
            TranslateObject(clonedObj, offsetX, offsetY);
        to.PageObjects.Add(clonedObj);
    }
}

private static void TranslateObject(PdfPageObject clonedObj, float offsetX, float offsetY)
{
    clonedObj.TransformClipPath(new FS_MATRIX(1, 0, 0, 1, offsetX, offsetY));
    if (clonedObj is PdfTextObject)
    {
        var m = (clonedObj as PdfTextObject).Matrix;
        m.Translate(offsetX, offsetY);
        (clonedObj as PdfTextObject).Matrix = m;
    }
    else if (clonedObj is PdfPathObject)
    {
        var m = (clonedObj as PdfPathObject).Matrix;
        m.Translate(offsetX, offsetY);
        (clonedObj as PdfPathObject).Matrix = m;
    }
    else if (clonedObj is PdfImageObject)
    {
        var m = (clonedObj as PdfImageObject).Matrix;
        m.Translate(offsetX, offsetY);
        (clonedObj as PdfImageObject).Matrix = m;
    }
    else if (clonedObj is PdfShadingObject)
    {
        var m = (clonedObj as PdfShadingObject).Matrix;
        m.Translate(offsetX, offsetY);
        (clonedObj as PdfShadingObject).Matrix = m;
    }
    else if (clonedObj is PdfFormObject)
    {
        var m = (clonedObj as PdfFormObject).Matrix;
        m.Translate(offsetX, offsetY);
        (clonedObj as PdfFormObject).Matrix = m;
    }
}