Request Attachment is not working with Node Express

Question

So I'm trying to build an endpoint to use in my client app to download a file using express, and here is function:

downloadInvoice: async (req, res) => {
 res.attachment(__dirname + '/invoices/a.pdf');
 res.send();
},

And yes, when I try to get the file that is on that path, it downloads me a pdf but that is not opening, with the error message: Failed to load PDF document.