Many projects require converting Excel files to PDF format, whether generating reports, sharing data, or creating documents. Initially, I, like many developers, thought this would be an easily automated task. However, the journey to find a free and reliable solution is fraught with challenges: limitations, compatibility issues, and expensive commercial tools.

Eventually, I overcame these difficulties and built my own Excel to PDF converter and made it available as an open source tool to other developers who may face the same dilemma.

Frustrating experience

Business Tools

Initial search results pointed to paid solutions such as Aspose.Cells, Syncfusion and others. While they are powerful, their license fees are high, making them cost-prohibitive for small or personal projects.

Online Services

Free online converters seem to be a good choice, but they are not suitable for automation. These tools often have privacy issues (because files are uploaded to third-party servers), file size limitations, and do not provide programming APIs.

Open Source Library

I also explored open source libraries, but most lacked functionality to convert Excel files to PDF. Even those libraries that do have this feature are often unreliable or don't support modern Microsoft Office formats.

LibreOffice in headless mode

After a few weeks of searching, I stumbled upon a way to use LibreOffice in headless mode. LibreOffice is a free, open-source office suite that can convert multiple file formats, including Excel, to PDF. When running in headless mode, it operates via the command line, making it ideal for automation.

How my solution works

In order to make it easier for developers to use, I built a lightweight Go-based HTTP server that acts as a REST API. This server encapsulates the functionality of LibreOffice and allows any programming language to interact with it via HTTP requests.

Main functions

1. Supports multiple file formats: Supports .xlsx, .xls, .csv, .docx, .pptx and other formats.
2. Auto Cleanup: Temporary files are automatically deleted after one hour to save disk space.
3. Custom Fonts: Custom fonts can be installed by cloning the GitHub repository or using a Docker volume.
4. Cross-language integration: Works with any programming language that supports HTTP.

Temporary directory method

Instead of relying on the system's temporary directory, I chose to use a custom ./tmp directory. This ensures consistent behavior since system temporary directories sometimes have unpredictable permissions.

Implementation details

Workflow

1. File upload: The client uses the /convert endpoint to upload Excel files through POST requests.
2. Temporary Storage: The server saves files to the ./tmp directory with file names based on timestamps.
3. Convert: Call LibreOffice in headless mode to convert the file to PDF and save the result to the same directory.
4. File cleaning: Background goroutine deletes files older than one hour.
5. Response: Return the converted PDF as an HTTP response.

Getting Started Guide

GitHub repository

You can find the source code at http://www.miracleart.cn/link/5b1add8961a1cfa07e60838ffd0f83e7.

Docker image

This project also provides Docker image: wteja/pdf-converter.

Run Docker container

<code>docker pull wteja/pdf-converter
docker run -p 5000:5000 wteja/pdf-converter</code>

Examples of integration with other languages

Since the service is exposed over HTTP, you can interact with it using any programming language.

C#

var client = new HttpClient();
var fileContent = new ByteArrayContent(File.ReadAllBytes("example.xlsx"));
var formData = new MultipartFormDataContent { { fileContent, "file", "example.xlsx" } };

var response = await client.PostAsync("http://localhost:5000/convert", formData);
var pdfBytes = await response.Content.ReadAsByteArrayAsync();
File.WriteAllBytes("output.pdf", pdfBytes);

Node.js

const axios = require("axios");
const FormData = require("form-data");
const fs = require("fs");

const form = new FormData();
form.append("file", fs.createReadStream("example.xlsx"));

axios.post("http://localhost:5000/convert", form, { headers: form.getHeaders() })
  .then(response => fs.writeFileSync("output.pdf", response.data))
  .catch(console.error);

Python

import requests

with open("example.xlsx", "rb") as f:
    response = requests.post("http://localhost:5000/convert", files={"file": f})

with open("output.pdf", "wb") as f:
    f.write(response.content)

Go

package main

import (
    "bytes"
    "io"
    "mime/multipart"
    "net/http"
    "os"
)

func main() {
    file, _ := os.Open("example.xlsx")
    defer file.Close()

    body := &bytes.Buffer{}
    writer := multipart.NewWriter(body)
    part, _ := writer.CreateFormFile("file", "example.xlsx")
    io.Copy(part, file)
    writer.Close()

    req, _ := http.NewRequest("POST", "http://localhost:5000/convert", body)
    req.Header.Set("Content-Type", writer.FormDataContentType())

    resp, _ := http.DefaultClient.Do(req)
    defer resp.Body.Close()

    out, _ := os.Create("output.pdf")
    defer out.Close()
    io.Copy(out, resp.Body)
}

Challenges and Tradeoffs

Image size

Due to LibreOffice dependencies, the Docker image size is 2.67 GB. Although I tested smaller images such as Alpine, they contained older versions of LibreOffice that were not compatible with modern Microsoft Office formats. Although Debian provides the latest LibreOffice, the generated image is larger (about 3 GB).

Why it’s worth it

The larger image size is an acceptable trade-off compared to the cost of commercial solutions. Once set up, the image can be reused in multiple projects without paying additional license fees.

Conclusion

Frustration in finding a free Excel to PDF converter led me to build my own solution using LibreOffice in headless mode. While it's not perfect, it's free, reliable, and flexible. If you face the same challenge, I hope this project saves you time and effort.

Please check out the project on GitHub or pull the Docker image from Docker Hub. Please let me know how it works for you, or if you have suggestions for improvements.

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