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。到此为止，我能够将30
MB的文件缩小到6 MB左右。
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，但是它可以工作并保持PDF / A-1b兼容性。
