如何使用 pdfbox 或其他 java 库减小合并的 PDF/A-1b 文件的大小 [英] How to reduce the size of merged PDF/A-1b files with pdfbox or other java library

问题描述

输入:带有嵌入字体的(例如 14 个)PDF/A-1b 文件的列表.
处理:与 Apache PDFBOX 进行简单的合并.
结果:1 个 PDF/A-1b 文件，文件过大(过大).(几乎是所有源文件大小的总和).

Input: A list of (e.g. 14) PDF/A-1b files with embedded fonts.
Processing: Doing a simple merge with Apache PDFBOX.
Result: 1 PDF/A-1b file with large (too large) file size. (It is almost the sum of the size of all the source files).

问题:有没有办法减小生成的 PDF 的文件大小?
想法:删除多余的嵌入字体.但是怎么办?这是正确的做法吗?

Question: Is there a way to reduce the file size of the resulting PDF?
Idea: Remove redundant embedded fonts. But how to? And is it the right way to do?

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.

Unfortunately the following code is not doing the job, but is highlighting the obvious problem.

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("	" + 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:

The code produces an output like that:

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

感谢任何帮助......

Any help appreciated ...

推荐答案

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，将字典中的实际字体文件项替换为已查看的字体文件项，引用已被删除并且可以进行压缩.这样，我就能够将 30 MB 的文件缩小到 6 MB 左右.

When debugging in the file, I recognized that the font files for the same fonts were referenced several times. So replacing the actual font file item in the dictionary with an already viewed font file item, the reference was removed and compression could be done. By that, I was able to shrink a 30 MB File to around 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法，但它可以工作并保持 PDF/A-1b 兼容性.

Maybe this is not the most elegant way to do that, but it works and keeps the PDF/A-1b compatibility.

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