将PDF页面大小转为A4格式---itextPdf实现

<!-- https://mvnrepository.com/artifact/com.itextpdf/itextpdf -->
        <dependency>
            <groupId>com.itextpdf</groupId>
            <artifactId>itextpdf</artifactId>
            <version>5.5.13.2</version>
        </dependency>

package com.fan.study.common.util;

import com.itextpdf.text.Document;
import com.itextpdf.text.PageSize;
import com.itextpdf.text.pdf.PdfContentByte;
import com.itextpdf.text.pdf.PdfImportedPage;
import com.itextpdf.text.pdf.PdfReader;
import com.itextpdf.text.pdf.PdfWriter;

import java.awt.geom.AffineTransform;
import java.io.FileOutputStream;

public class PdfConvertA4Utils {
    /**
     * @Param source 源文件
     * @Param target 转换后文件
     * @Description 将PDF转为A4格式
     * @Date: 2021/4/25
     **/
    private static void convert(String source, String target) {
        try {
            PdfReader pdfReader = new PdfReader(source);
            Document doc = new Document();
            PdfWriter writer = PdfWriter.getInstance(doc, new FileOutputStream(target));
            doc.open();
            PdfContentByte cb = writer.getDirectContent();
            for(int i = 1; i <= pdfReader.getNumberOfPages(); i++){
                PdfImportedPage page = writer.getImportedPage(pdfReader, i);
                float width = page.getWidth();
                float height = page.getHeight();
                if(height > width) {
                    //横向
                    doc.setPageSize(PageSize.A4);
                    doc.newPage();
                    //计算比例
                    float widthScale = getWidthScale(width);
                    float heightScale = getHeightScale(height);
                    //addTemplate方法中有6个float类型的参数，是通过二维图像仿射变换得到的
                    //cb.addTemplate(page, new AffineTransform(widthScale, 0, 0, heightScale,0,0));
                    //二维图像仿射变换:https://www.cnblogs.com/v2m_/archive/2013/05/09/3070187.html
                    cb.addTemplate(page, widthScale, 0, 0, heightScale,0,0);
                } else {
                    //纵向
                    doc.setPageSize(new com.itextpdf.text.Rectangle(PageSize.A4.getHeight(), PageSize.A4.getWidth()));
                    doc.newPage();
                    float widthScale = getWidthScale(height);
                    float heightScale = getHeightScale(width);
                    cb.addTemplate(page, widthScale, 0, 0, heightScale,0,0);
                }
            }
            doc.close();
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
    private static float getWidthScale(float width) {
        float scale = PageSize.A4.getWidth() / width;
        return scale;
    }

    private static float getHeightScale(float height) {
        float scale = PageSize.A4.getHeight() / height;
        return scale;
    }
}