public IActionResult Compress()

{

    var inputFilePath = @"C:\input.pdf";

    var outputFilePath = @"C:\output.pdf";

    CompressPdf(inputFilePath, outputFilePath);

    return File(outputFilePath, "application/pdf");

}

Install-Package iTextSharp-LGPL

public void CompressPdf(string inputFilePath, string outputFilePath)

{

    using (var inputStream = new FileStream(inputFilePath, FileMode.Open))

    {

        using (var outputStream = new FileStream(outputFilePath, FileMode.Create))

        {

            var reader = new PdfReader(inputStream);

            var pdfStamper = new PdfStamper(reader, outputStream, PdfWriter.VERSION_1_5);

            pdfStamper.SetFullCompression();

            pdfStamper.Close();

        }

    }

}