I can not modify and save my pdf file

[thomasop]

Hello,
I use for my website next js.
I have a pdf on my site and I duplicate this pdf. And I want to edit the copy of pdf and save this. But it doesn't work.
For duplicate my pdf is use this (it work well) :

          copyFile(
            "public/assets/pdf/test.pdf",
            `public/assets/pdf/copy.pdf`,
            (err) => {
                if (err) throw err;
              console.log("source.txt was copied to destination.txt");
            }
          );

And I try to modify and save (it not work) :

          const url = `http://localhost:3000/assets/pdf/copy.pdf`;
          const existingPdfBytes = await fetch(url).then((res) =>
            res.arrayBuffer()
          );

          const pdfDoc = await PDFDocument.load(existingPdfBytes);
          const helveticaFont = await pdfDoc.embedFont(StandardFonts.Helvetica);

          const pages = pdfDoc.getPages();

          const firstPage = pages[0];
          const { width, height } = firstPage.getSize();
          firstPage.drawText("This is a test", {
            x: 25,
            y: height / 2 + 300,
            size: 20,
            font: helveticaFont,
            color: rgb(0.95, 0.1, 0.1),
            rotate: degrees(0),
          });
          const pdfBytes = await pdfDoc.save();

I try to do this in frontend and backend but nothing work.
The pdf "copy.pdf" is not edit and not save when I open it. So how to save edit pdf in folder ?
I follow this tutorial https://jsfiddle.net/Hopding/64zajhge/1/

Thank you for your help