```go title="Go"
package main

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

func main() {
	file, err := os.Open("document.pdf")
	if err != nil {
		log.Fatalf("failed to open file: %v", err)
	}
	defer file.Close()

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

	resp, err := http.Post("http://localhost:8000/extract", writer.FormDataContentType(), body)
	if err != nil {
		log.Fatalf("request failed: %v", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		var errResp map[string]string
		json.NewDecoder(resp.Body).Decode(&errResp)
		log.Fatalf("error: %s: %s", errResp["error_type"], errResp["message"])
	}

	var result map[string]interface{}
	json.NewDecoder(resp.Body).Decode(&result)
	println("Success:", result["content"].(string))
}
```