how to convert word to pdf with openxml?

You could use the Microsoft Office Interop libraries to automate Microsoft Word to open the document and then save it as a PDF. However, this approach requires Microsoft Word to be installed on the machine running the code, and it's generally not recommended for server-side applications due to potential reliability and licensing issues.

https://stackoverflow.com/questions/607669/how-do-i-convert-word-files-to-pdf-programmatically

its not easy to found the proper library but i found this and its working i hope its usefull to you

download this library manually and install in nuget package

then store this in "C:\Program Files (x86)\Microsoft SDKs\NuGetPackages"itext.8.0.1.nupkg
itext.bouncy-castle-adapter.8.0.1.nupkg
its availble on this path 
https://github.com/itext/itext7-dotnet/releases/tag/8.0.1

private readonly ILogger _logger;
private readonly string wordDocumentPath = "WordDocuments/SampleWordDocument.docx"; // Provide the actual path to your Word document.
private readonly string pdfFilePath = @"D:\CoreDemoWordtoPdf\WordDocToPdf\WordDocToPdf\PdfFiles\Output.pdf";

public IActionResult ConvertToPdf()
{
    string pdfFileName = "ConvertedDocument.pdf";
    string pdfFilePath = Path.Combine(Directory.GetCurrentDirectory(), "PdfFiles", pdfFileName);

    using (MemoryStream wordStream = new MemoryStream(System.IO.File.ReadAllBytes(wordDocumentPath)))
    {
        bool conversionResult = WordToPdfConverter.ConvertWordToPdf(wordStream, pdfFilePath);

        if (conversionResult)
        {
            byte[] pdfBytes = System.IO.File.ReadAllBytes(pdfFilePath);
            return File(pdfBytes, "application/pdf", pdfFileName);
        }
        else
        {
            return Content("Word to PDF conversion failed.");
        }
    }
}

public static bool ConvertWordToPdf(Stream wordStream, string pdfFilePath)
{
    using (WordprocessingDocument wordDocument = WordprocessingDocument.Open(wordStream, false))
    {
        using (var pdfWriter = new PdfWriter(pdfFilePath))
        {
            using (var pdfDocument = new PdfDocument(pdfWriter))
            {
                using (var document = new iText.Layout.Document(pdfDocument))
                {
                    foreach (var paragraph in wordDocument.MainDocumentPart.Document.Body.Elements())
                    {
                        string text = paragraph.InnerText;
                        document.Add(new iText.Layout.Element.Paragraph(text));
                    }
                }
            }
        }
    }

    return true;
}

i hope this is usefull