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:包含嵌入式字体的(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PDF/A-1b文件，文件大小太大(太大). (这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:是否可以减小生成的PDF的文件大小?
想法:删除多余的嵌入式字体.但是怎么办呢?这是正确的方法吗?

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，以下代码无法完成工作，但是突出了明显的问题.

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("\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:

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

任何帮助表示赞赏...

Any help appreciated ...

推荐答案

在文件中进行调试时，我认识到多次引用相同字体的字体文件.因此，用已经查看过的字体文件项替换字典中的实际字体文件项，即可删除引用并进行压缩.这样，我可以将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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