如何使用pdfbox或其他java库减小合并的PDF / A-1b文件的大小

2021-07-20 13:07:43

输入：包含嵌入字体的(例如14个)PDF / A-1b文件列表.
处理：与Apache PDFBOX进行简单合并.
结果：1个文件大小(太大)的PDF / A-1b文件. (它几乎是所有源文件大小的总和).

问题：有没有办法减少生成的PDF的文件大小？
想法：删除冗余的嵌入字体.但是怎么样？这是正确的方法吗？

不幸的是,以下代码没有完成这项工作,但突出了明显的问题.

try (PDDocument document = PDDocument.load(new File("E:/tmp/16189_ZU_20181121195111_5544_2008-12-31_Standardauswertung.pdf"))) {
    List<COSName> collectedFonts = new ArrayList<>();
    PDPageTree pages = document.getDocumentCatalog().getPages();
    int pageNr = 0;
    for (PDPage page : pages) {
        pageNr++;
        Iterable<COSName> names = page.getResources().getFontNames();
        System.out.println("Page " + pageNr);
        for (COSName name : names) {
            collectedFonts.add(name);
            System.out.print("\t" + name + " - ");
            PDFont font = page.getResources().getFont(name);
            System.out.println(font + ", embedded: " + font.isEmbedded());
            page.getCOSObject().removeItem(COSName.F);
            page.getResources().getCOSObject().removeItem(name);
        }
    }
    document.save("E:/tmp/output.pdf");
}

代码产生如下输出：

Page 1
    COSName{F23} - PDTrueTypeFont ArialMT-Bold, embedded: true
    COSName{F27} - PDTrueTypeFont ArialMT-Regular, embedded: true
Page 2
    COSName{F23} - PDTrueTypeFont ArialMT-Bold, embedded: true
    COSName{F33} - PDTrueTypeFont ArialMT-BoldItalic, embedded: true
    COSName{F25} - PDTrueTypeFont ArialMT-Italic, embedded: true
    COSName{F27} - PDTrueTypeFont ArialMT-Regular, embedded: true
Page 3
    COSName{F23} - PDTrueTypeFont ArialMT-Bold, embedded: true
    COSName{F25} - PDTrueTypeFont ArialMT-Italic, embedded: true
    COSName{F27} - PDTrueTypeFont ArialMT-Regular, embedded: true
Page 4
    COSName{F23} - PDTrueTypeFont ArialMT-Bold, embedded: true
    COSName{F25} - PDTrueTypeFont ArialMT-Italic, embedded: true
    COSName{F27} - PDTrueTypeFont ArialMT-Regular, embedded: true
Page 5
    COSName{F23} - PDTrueTypeFont ArialMT-Bold, embedded: true
    COSName{F33} - PDTrueTypeFont ArialMT-BoldItalic, embedded: true
    COSName{F27} - PDTrueTypeFont ArialMT-Regular, embedded: true
Page 6
    COSName{F23} - PDTrueTypeFont ArialMT-Bold, embedded: true
    COSName{F33} - PDTrueTypeFont ArialMT-BoldItalic, embedded: true
    COSName{F27} - PDTrueTypeFont ArialMT-Regular, embedded: true
Page 7
    COSName{F23} - PDTrueTypeFont ArialMT-Bold, embedded: true
    COSName{F33} - PDTrueTypeFont ArialMT-BoldItalic, embedded: true
    COSName{F27} - PDTrueTypeFont ArialMT-Regular, embedded: true
Page 8
    COSName{F23} - PDTrueTypeFont ArialMT-Bold, embedded: true
    COSName{F25} - PDTrueTypeFont ArialMT-Italic, embedded: true
    COSName{F27} - PDTrueTypeFont ArialMT-Regular, embedded: true
Page 9
    COSName{F23} - PDTrueTypeFont ArialMT-Bold, embedded: true
    COSName{F33} - PDTrueTypeFont ArialMT-BoldItalic, embedded: true
    COSName{F25} - PDTrueTypeFont ArialMT-Italic, embedded: true
    COSName{F27} - PDTrueTypeFont ArialMT-Regular, embedded: true
Page 10
    COSName{F23} - PDTrueTypeFont ArialMT-Bold, embedded: true
    COSName{F33} - PDTrueTypeFont ArialMT-BoldItalic, embedded: true
    COSName{F25} - PDTrueTypeFont ArialMT-Italic, embedded: true
    COSName{F27} - PDTrueTypeFont ArialMT-Regular, embedded: true
Page 11
    COSName{F23} - PDTrueTypeFont ArialMT-Bold, embedded: true
    COSName{F33} - PDTrueTypeFont ArialMT-BoldItalic, embedded: true
    COSName{F27} - PDTrueTypeFont ArialMT-Regular, embedded: true
Page 12
    COSName{F23} - PDTrueTypeFont ArialMT-Bold, embedded: true
    COSName{F25} - PDTrueTypeFont ArialMT-Italic, embedded: true
    COSName{F27} - PDTrueTypeFont ArialMT-Regular, embedded: true
Page 13
    COSName{F23} - PDTrueTypeFont ArialMT-Bold, embedded: true
    COSName{F25} - PDTrueTypeFont ArialMT-Italic, embedded: true
    COSName{F27} - PDTrueTypeFont ArialMT-Regular, embedded: true
Page 14
    COSName{F23} - PDTrueTypeFont ArialMT-Bold, embedded: true
    COSName{F25} - PDTrueTypeFont ArialMT-Italic, embedded: true
    COSName{F27} - PDTrueTypeFont ArialMT-Regular, embedded: true

任何帮助赞赏…

解决方法:

在文件中调试时,我发现相同字体的字体文件被多次引用.因此,使用已查看的字体文件项替换字典中的实际字体文件项,删除了引用并可以进行压缩.通过这种方式,我能够将30 MB的文件缩小到大约6 MB.

    File file = new File("test.pdf");

    PDDocument doc = PDDocument.load(file);
    Map<String, COSBase> fontFileCache = new HashMap<>();
    for (int pageNumber = 0; pageNumber < doc.getNumberOfPages(); pageNumber++) {
        final PDPage page = doc.getPage(pageNumber);
        COSDictionary pageDictionary = (COSDictionary) page.getResources().getCOSObject().getDictionaryObject(COSName.FONT);
        for (COSName currentFont : pageDictionary.keySet()) {
            COSDictionary fontDictionary = (COSDictionary) pageDictionary.getDictionaryObject(currentFont);
            for (COSName actualFont : fontDictionary.keySet()) {
                COSBase actualFontDictionaryObject = fontDictionary.getDictionaryObject(actualFont);
                if (actualFontDictionaryObject instanceof COSDictionary) {
                    COSDictionary fontFile = (COSDictionary) actualFontDictionaryObject;
                    if (fontFile.getItem(COSName.FONT_NAME) instanceof COSName) {
                        COSName fontName = (COSName) fontFile.getItem(COSName.FONT_NAME);
                        fontFileCache.computeIfAbsent(fontName.getName(), key -> fontFile.getItem(COSName.FONT_FILE2));
                        fontFile.setItem(COSName.FONT_FILE2, fontFileCache.get(fontName.getName()));
                    }
                }
            }
        }
    }

    final ByteArrayOutputStream baos = new ByteArrayOutputStream();
    doc.save(baos);
    final File compressed = new File("test_compressed.pdf");
    baos.writeTo(new FileOutputStream(compressed));

也许这不是最优雅的方式,但它可以工作并保持PDF / A-1b的兼容性.

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