第一步，pom.xml中引入jar包 

        <dependency>
            <groupId>org.apache.pdfbox</groupId>
            <artifactId>pdfbox</artifactId>
            <version>2.0.24</version>
        </dependency>

 第二步

package org.example.test.example.changefile;

import org.apache.pdfbox.pdmodel.PDDocument;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.util.Base64;

public class ConvertPDFToBase64String {
    public static void main(String[] args) {
        String inputPdfPath = "C:\\Users\\EDY\\Desktop\\input.pdf"; // 输入PDF文件的路径
        String outputTxtPath = "C:\\Users\\EDY\\Desktop\\output.txt"; // 输出TXT文件的路径

        try {
            // 加载PDF文档
            try (PDDocument document = PDDocument.load(new File(inputPdfPath))) {
                // 创建字节输出流
                ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
                // 将PDF文档写入字节输出流
                document.save(byteArrayOutputStream);
                // 获取PDF文档的字节数据
                byte[] pdfBytes = byteArrayOutputStream.toByteArray();

                // 使用Base64编码PDF字节数据
                String base64EncodedString = Base64.getEncoder().encodeToString(pdfBytes);

                // 将Base64编码的字符串写入TXT文件
                try (FileWriter writer = new FileWriter(outputTxtPath)) {
                    writer.write(base64EncodedString);
                }

                System.out.println("PDF file has been converted to Base64 string and saved to TXT file.");
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}