Here is C# code that shows how to convert DOCX document to a PDF document: 
/// <summary>
/// Converts DOCX document to a PDF document using ImageCollection and PdfEncoder classes.
/// </summary>
public static void ConvertDocxToPdf(string docxFileName, string pdfFileName)
{
    // specify that VintaSoft Imaging .NET SDK should use GDI+ for drawing of 2D graphics
    Vintasoft.Imaging.Drawing.Gdi.GdiGraphicsFactory.SetAsDefault();
    // specify that VintaSoft Imaging .NET SDK should use SkiaSharp for drawing of 2D graphics
    //Vintasoft.Imaging.Drawing.SkiaSharp.SkiaSharpDrawingFactory.SetAsDefault();

    // create image collection
    using (Vintasoft.Imaging.ImageCollection imageCollection = new Vintasoft.Imaging.ImageCollection())
    {
        // add DOCX document to collection
        imageCollection.Add(docxFileName);

        // create pdfEncoder
        using (Vintasoft.Imaging.Codecs.Encoders.PdfEncoder pdfEncoder = 
            new Vintasoft.Imaging.Codecs.Encoders.PdfEncoder(true))
        {
            // set comression for image resources
            pdfEncoder.Settings.Compression = Vintasoft.Imaging.Codecs.Encoders.PdfImageCompression.Jpeg;

            // save images of image collection to PDF document using PdfEncoder
            imageCollection.SaveSync(pdfFileName, pdfEncoder);
        }

        // dispose images
        imageCollection.ClearAndDisposeItems();
    }
}