Simple sample code to convert a docx-file to pdf using Aspose.Words. The relevant part is the method SaveAsPdf(). The Nuget package for Aspose.Words needs to be added to the project.

ConvertDocX2PdfWithAspose.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using System.IO;

using Aspose.Words;
using Aspose.Pdf;

namespace PDFToolsTester
{
    class Program
    {
        static void Main(string[] args)
        {
            string docxFilename = "C:\\Temp\\MyDocument.docx";
            string pdfFilename = "C:\\Temp\\MyDocument.pdf";

            using (FileStream fs = new FileStream(docxFilename, FileMode.Open))
            {
                MemoryStream msPdf = SaveAsPdf(fs);

                if (msPdf != null)
                {
                    using (FileStream fsPdf = new FileStream(pdfFilename, FileMode.Create))
                    {
                        msPdf.CopyTo(fsPdf, (int)msPdf.Length);

                        fsPdf.Flush();
                        fsPdf.Close();
                    }

                    msPdf.Dispose();
                }
            }
        }

        static MemoryStream SaveAsPdf(Stream fs)
        {
            MemoryStream result = null;

            byte[] buffer = new byte[fs.Length];

            fs.Read(buffer, 0, (int)fs.Length);

            using (MemoryStream ms = new MemoryStream(buffer, false))
            {
                Aspose.Words.Document document = new Aspose.Words.Document(ms);

                using (MemoryStream msPdf = new MemoryStream())
                {
                    document.Save(msPdf, Aspose.Words.SaveFormat.Pdf);

                    msPdf.Position = 0;

                    result = new MemoryStream((int)msPdf.Length);

                    msPdf.CopyTo(result);

                    result.Position = 0;
                }
            }

            return result;
        }
    }
}