通过Node.js API借助FTP从远程服务器下载PDF的问题

Got it, let's figure out why your PDF download isn't working! The main issue here is that you're treating a binary PDF file like a string—and that's breaking the data irreparably. PDFs are binary formats, so every byte matters. Converting them to a string (with toString()) uses text encoding (like UTF-8) that mangles raw binary values, resulting in a corrupted file when you try to download it.

Here's how to fix this step by step:

1. Stop using strings—use Buffers for binary data #

Instead of concatenating strings, collect the raw binary chunks into an array of Buffers, then merge them into a single Buffer once the transfer is complete. This preserves the original PDF data.

2. Set proper HTTP headers for download #

When sending the PDF to your website's frontend, you need to tell the browser exactly what it's receiving and that it should trigger a download (not try to display it as text).

Full working example (using Express as an example framework) #

// Assuming you're using Express for your API
const express = require('express');
const app = express();
const FTP = require('jsftp');

// Initialize your FTP client (fill in your credentials)
const ftpClient = new FTP({
  host: 'your-remote-server.com',
  user: 'your-ftp-username',
  pass: 'your-ftp-password'
});

app.get('/download-pdf', (req, res) => {
  const pdfFilePath = 'path/to/my/file';
  let binaryChunks = [];

  ftpClient.get(pdfFilePath, (err, socket) => {
    if (err) {
      console.error("Failed to start fetching PDF:", err);
      return res.status(500).send("Could not retrieve PDF file");
    }

    // Collect raw binary chunks
    socket.on('data', (chunk) => binaryChunks.push(chunk));

    socket.on('close', (err) => {
      if (err) {
        console.error("Error during PDF transfer:", err);
        return res.status(500).send("Error downloading PDF");
      }

      // Merge chunks into a single Buffer
      const pdfBuffer = Buffer.concat(binaryChunks);

      // Set headers to trigger download
      res.setHeader('Content-Type', 'application/pdf');
      res.setHeader(
        'Content-Disposition',
        'attachment; filename="my-document.pdf"' // Replace with your desired filename
      );

      // Send the PDF data to the browser
      res.send(pdfBuffer);
    });

    // Handle socket errors
    socket.on('error', (err) => {
      console.error("Socket error:", err);
      res.status(500).send("Connection error during download");
    });

    socket.resume();
  });
});

app.listen(3000, () => console.log("API running on port 3000"));

Bonus: Optimize for large PDFs (streaming) #

If your PDF is large, collecting all chunks in memory isn't efficient. Instead, you can pipe the FTP socket directly to the HTTP response to stream the data in real-time:

app.get('/download-pdf', (req, res) => {
  const pdfFilePath = 'path/to/my/file';

  ftpClient.get(pdfFilePath, (err, socket) => {
    if (err) {
      return res.status(500).send("Could not retrieve PDF file");
    }

    // Set headers first
    res.setHeader('Content-Type', 'application/pdf');
    res.setHeader('Content-Disposition', 'attachment; filename="my-document.pdf"');

    // Pipe the FTP socket directly to the response
    socket.pipe(res);

    socket.on('error', (err) => {
      console.error("Stream error:", err);
      res.status(500).send("Error streaming PDF");
    });

    socket.resume();
  });
});

This avoids loading the entire file into memory, which is better for performance and server resources.

内容的提问来源于stack exchange，提问作者jean-max