How to remove Spire.XLS signature from pdf File?

I'm working on java, i have an excel file named "file.xlsx" that i want to convert to pdf using Spire.XLS , this is the code i wrote for that :

            //Create a Workbook instance
            workbook = new Workbook();
            //Load an Excel file
            workbook.loadFromFile("file.xlsx");

            //Get the first worksheet
            sheet = workbook.getWorksheets().get(0);
            //Add a new worksheet to the workbook
            sheet_copy = workbook.getWorksheets().add("Copy of sheet1");
            //Copy the first worksheet to the new worksheet
            sheet_copy.copyFrom(sheet);

            //Save the result file
            workbook.saveToFile("file.xlsx");
            
            workbook.saveToFile("result.pdf", com.spire.xls.FileFormat.PDF);

The pdf file is generated but with a signature : Evaluation Warning : The document was created with Spire.XLS for Java , i'm wondering about how to remove it , any idea ?