使用Tika-App 1.21提取PDF文本，如何处理无Unicode映射字符？

Got it, let's break this down—dealing with non-Unicode-mapped characters in PDFs with Tika 1.21 is a common pain point, especially with custom or legacy fonts. Here's how to tackle both extracting these characters and finding their Unicode equivalents:

First, let's focus on getting those characters out of the PDF in the first place:

• Upgrade Tika (if possible) : Tika 1.21 is pretty old (released in 2019), and newer versions (2.x+) have major improvements in font handling and Unicode mapping. Upgrading might automatically resolve many of these issues without extra work.
• Enable font extraction and analysis : When running Tika-App, use the --extract-fonts flag to pull the actual font files from the PDF. This lets you inspect the font's internal mappings later. For example:

  java -jar tika-app-1.21.jar --extract-fonts your_file.pdf ./extracted_fonts
  

• Force raw text extraction with underlying tools : Tika uses PDFBox under the hood. You can try bypassing some of Tika's abstraction by using PDFBox directly (or adjusting Tika's PDF parser settings) to extract raw character codes. Alternatively, use pdftotext (from the Poppler toolkit) with the -raw flag to get unprocessed character data, which might preserve the original codes even if they don't map to Unicode yet.
• Enable OCR as a fallback : If the characters are actually rendered as images (not text), Tika can integrate with Tesseract OCR to extract them. Run Tika with the --ocr flag to trigger OCR for image-based content:

  java -jar tika-app-1.21.jar --ocr your_file.pdf output.txt
  

Once you have access to the character data or font files, here's how to find their Unicode matches:

• Inspect the font with FontForge : Open the extracted font file (from the --extract-fonts step) in FontForge. You can view each glyph's Unicode mapping (if any), or look at the glyph's name (many fonts use standard naming conventions that map to Unicode). For example, a glyph named frac12 likely corresponds to the Unicode character ½ (U+00BD).
• Check the PDF's ToUnicode table : PDFs often include a ToUnicode map that maps font-specific codes to Unicode. Use pdffonts (from Poppler) to check if the font has this table:

  pdffonts your_file.pdf
  

  Look for the "ToUnicode" column—if it says "yes", you can extract this table using PDFBox's command-line tools or custom code to map the raw character codes to Unicode.
• Use OCR for visual matching : If you can't map via font data, take a screenshot of the problematic character and run it through Tesseract OCR. Tesseract is good at identifying visual equivalents even if the original PDF doesn't have text mappings.
• Check private use area (PUA) mappings : Some fonts use Unicode's Private Use Areas (PUAs) for custom characters. While these aren't standard, you can look up industry-specific mappings (e.g., for technical symbols or regional scripts) to find standard Unicode equivalents that were added in later Unicode versions.
• Compare with similar glyphs : If no direct match exists, look for visually similar Unicode characters or combining sequences. For example, a custom "strikethrough a" might map to ą̶ (U+0105 + U+0336) if it doesn't have its own code.

内容的提问来源于stack exchange，提问作者Himanshu Khandelwal