Electron 8 printToPdf pageSize always measures the same

This is my code to export to PDF in Electron 8.1.0 on Windows 10:

ipcMain.on('export-pdf', (event, pdfPath) => {
        const BrowserWindow = require('electron').BrowserWindow;
        const win = BrowserWindow.getFocusedWindow();

        win.webContents.printToPDF({
            marginsType: 2,
            printBackground: false,
            printSelectionOnly: false,
            landscape: false,
            pageSize: 'Letter',
            scaleFactor: 100
        }).then(data => {
            fs.writeFile(`${pdfPath}.pdf`, data, (error) => {
                if (error) throw error;
                event.sender.send('export-pdf-end', true);
            })
        }).catch(error => {
            logger.error(`Error at save PDF in ${pdfPath}`, error);
            event.sender.send('export-pdf-end', false);
        });
    });

Everything works fine except the page size. Regardless of the value that I configure, the generated pdf always measures 11.69in x 16.54in and I need the exact letter format (8.5in x 11in). I have tried all the options including height and width in microns but it doesn't work.

UPDATE

I tried Electron 8.0.0 on Linux (Debian 10) and it worked perfect. I need to try the exact version on Linux and version 9 of Electron on Windows (when I have a better and cheaper Internet connection)

Any help? Thanks in advance.