static void Main(string[] args)
{
    using (var doc = PdfDocument.Load(@"e:\0\722-SWC-10.1.pdf"))
    {
        var page = doc.Pages[0];
        var contentsArray = ConvertContentsToArray(page);
        using (var form = PdfFormObject.Create(page))
        {
            //Add some objects to the form object
            form.PageObjects.Add(PdfTextObject.Create("Sample Text", 150, 150, PdfFont.CreateStock(doc, FontStockNames.Arial), 45.0f));
            //Create empty stream and add it to the list of indirect objects
            var stream = PdfTypeStream.Create();
            var list = PdfIndirectList.FromPdfDocument(doc);
            list.Add(stream);
            //Generate content of form object to that stream
            bool b = Pdfium.FPDF_GenerateContentToStream(doc.Handle, form.PageObjects.Handle, stream.Handle, IntPtr.Zero);
            //Add stream to the contentsArray
            contentsArray.AddIndirect(list, stream);
        }
        doc.Save(@"e:\0\722-SWC-10.1_modified.pdf", SaveFlags.NoIncremental);
    }
}

public static PdfTypeArray ConvertContentsToArray(PdfPage page)
{
    var pageDict = page.Dictionary;
    var list = PdfIndirectList.FromPdfDocument(page.Document);

    if (!pageDict.ContainsKey("Contents"))
    {
        var array = PdfTypeArray.Create();
        //Add array into list of indirect objects
        list.Add(array);
        //And set it as a contents of the page
        pageDict.SetIndirectAt("Contents", list, array);
        return array;
    }

    var contents = pageDict["Contents"];

    //check the original content whether it's an array
    if (contents is PdfTypeArray)
        return contents as PdfTypeArray;  //if contents is a array just return it
    else if (contents is PdfTypeIndirect)
    {
        if ((contents as PdfTypeIndirect).Direct is PdfTypeArray)
            return (contents as PdfTypeIndirect).Direct as PdfTypeArray; //if contents is a reference to array then return that array
        else if ((contents as PdfTypeIndirect).Direct is PdfTypeStream)
        {
            //if contents is a reference to a stream then create a new array and insert stream as a first element of array
            var array = PdfTypeArray.Create();
            array.AddIndirect(list, (contents as PdfTypeIndirect).Direct);
            //Add array into list of indirect objects
            list.Add(array);
            //And set it as a contents of the page
            pageDict.SetIndirectAt("Contents", list, array);
            return array;
        }
        else
            throw new Exception("Unexpected content type");
    }
    else if(contents is PdfTypeStream)
    {
        //if contents is a stream instead of reference to a stream then try to convert it to a reference then create a new array and insert stream as a first element of array
        list.Add(contents);
        var array = PdfTypeArray.Create();
        array.AddIndirect(list, contents);
        //Add array into list of indirect objects
        list.Add(array);
        //And set it as a contents of the page
        pageDict.SetIndirectAt("Contents", list, array);
        return array;
    }
    else 
        throw new Exception("Unexpected content type");
}

form.PageObjects.Add(PdfTextObject.Create("Sample Text", 150, 150, PdfFont.CreateStock(doc, FontStockNames.Arial), 45.0f));

var form = PdfFormObject.Create(page)

//Generate content of form object to that stream
if (!page.Dictionary.ContainsKey("Resources"))
    page.Dictionary["Resources"] = PdfTypeDictionary.Create();
bool b = Pdfium.FPDF_GenerateContentToStream(doc.Handle, tmpAnnot.NormalAppearance.Handle, stream.Handle, page.Dictionary["Resources"].As<PdfTypeDictionary>().Handle);

//Generate content of form object to that stream
PdfTypeDictionary resDict = FindResource(page.Dictionary);
bool b = Pdfium.FPDF_GenerateContentToStream(doc.Handle, tmpAnnot.NormalAppearance.Handle, stream.Handle, resDict.Handle);

private static PdfTypeDictionary FindResource(PdfTypeDictionary dict)
{
    if (dict.ContainsKey("Resources"))
        return dict["Resources"].As<PdfTypeDictionary>();

    if (dict.ContainsKey("Parent"))
        return FindResource(dict["Parent"].As<PdfTypeDictionary>());

    dict["Resources"] = PdfTypeDictionary.Create();
    return dict["Resources"].As<PdfTypeDictionary>();
}

Pdfium.FPDFPageObj_SetLineWidth(pdfPathObject.Handle, 10.0f);

			using (var doc = PdfDocument.Load(@"C:\temp\jv 755002_Limited (2).pdf"))
			{
				var page = doc.Pages[0];
				if (page.Annots == null) page.CreateAnnotations();
				var contentsArray = ConvertContentsToArray(page);
				using (var tmpAnnot = new PdfLinkAnnotation(page))
				{
					tmpAnnot.CreateEmptyAppearance(AppearanceStreamModes.Normal);
					tmpAnnot.NormalAppearance.Add(PdfTextObject.Create("Sample Text", 150, 150, PdfFont.CreateStock(doc, FontStockNames.Arial), 45.0f));

					var pdfTextObjects = (
						from pdfPageObject in page.PageObjects
						let pdfTextObject = pdfPageObject as PdfTextObject
						where pdfTextObject != null
						select pdfTextObject).ToList();
					foreach (var pdfTextObject in pdfTextObjects)
					{
						var text = pdfTextObject.TextUnicode;
						var boundingBox = pdfTextObject.BoundingBox;
						var x = boundingBox.left;
						var y = boundingBox.top;
						var w = boundingBox.Width;
						var h = boundingBox.Height;
						Console.WriteLine($"@({x}, {y}) {w} x {h} : {text}");

						var pdfPathObject = PdfPathObject.Create(FillModes.Winding, true);
						pdfPathObject.FillColor = new FS_COLOR(50, 200, 0, 0);
						pdfPathObject.StrokeColor = new FS_COLOR(50, 0, 100, 0);
						//Patagames.Pdf.Pdfium.FPDFPageObj_SetLineWidth(pdfPathObject.Handle, 1.15f);
						var d = 1;
						var bb = new FS_RECTF(x - d, y + d, x + w + d, y - h - d);
						pdfPathObject.Path.AppendRect(bb);
						pdfPathObject.CalcBoundingBox();
						tmpAnnot.NormalAppearance.Add(pdfPathObject);

						var pdfPathObjectNormalAppearance = PdfPathObject.Create(FillModes.Winding, false);
						pdfPathObjectNormalAppearance.FillColor = new FS_COLOR(50, 0, 200, 0);
						pdfPathObjectNormalAppearance.StrokeColor = new FS_COLOR(50, 100, 0, 0);
						pdfPathObjectNormalAppearance.Path.AppendRect(bb);
						pdfPathObjectNormalAppearance.CalcBoundingBox();
						var pdfTypeDictionary = PdfTypeDictionary.Create();
						pdfTypeDictionary["S"] = PdfTypeName.Create("URI");
						pdfTypeDictionary["URI"] = PdfTypeString.Create("https://google.com?" + text);
						var pdfLinkAnnotation = new PdfLinkAnnotation(page)
						{
							Rectangle = bb
						};
						pdfLinkAnnotation.Dictionary["A"] = pdfTypeDictionary;
						pdfLinkAnnotation.Dictionary["H"] = PdfTypeName.Create("N");
						var pdfTypeArray = PdfTypeArray.Create();
						pdfTypeArray.AddInteger(0);
						pdfTypeArray.AddInteger(0);
						pdfTypeArray.AddInteger(0);
						pdfLinkAnnotation.Dictionary["Border"] = pdfTypeArray;
						pdfLinkAnnotation.CreateEmptyAppearance(AppearanceStreamModes.Normal);
						pdfLinkAnnotation.NormalAppearance.Add(pdfPathObjectNormalAppearance);
						pdfLinkAnnotation.GenerateAppearance(AppearanceStreamModes.Normal);
						page.Annots.Add(pdfLinkAnnotation);
					}
					var stream = PdfTypeStream.Create();
					var list = PdfIndirectList.FromPdfDocument(doc);
					list.Add(stream);
					var resDict = FindResource(page.Dictionary);
					Patagames.Pdf.Pdfium.FPDF_GenerateContentToStream(doc.Handle, tmpAnnot.NormalAppearance.Handle, stream.Handle, resDict.Handle);
					//Add stream to the contentsArray
					contentsArray.AddIndirect(list, stream);
				}
				doc.Save(@"C:\temp\jv 755002_Limited-mod.pdf", SaveFlags.Incremental);
			}