Copy+pasting text from PDF results in garbage

Question

I am writing a Master's thesis - NLP system. I have one component - extractor.

It is extracting a plain text from PDF files. There are a few PDF files that can not be extracted correctly. Extractor (PDFBox library) returns a string like this:

"┤xDn║if|d├gDF"Ti&cD╬lh d FÁhis~n ╗xd f«"d┤ffih »h"

or

"10a61a91a22a25a3a27a17a23a20a8a13a14a61a25a17"

I was checking each file that makes this extraction's problem and all these files' text also can not be copy-pasted from PDF Reader (Adobe Reader and FoxIt reader). Viewing them in this readers is enabled, but after selecting its content and copying to the clipboard I get the same wrong text (as described above - strings of not semantically correct chars or strings of digits and letters).

Could anybody help me???

Answer 1

Very often in such cases, where you can't select, copy'n'paste text from the Acrobat (Reader) window, there is another option which may work nevertheless:

• Open 'File' menu,
• select 'Save as...',
• select 'Text (normal) (*.txt)',
• browse to the target directory,
• type the name you want to use for the text file.

You'll have all text from all pages in the file and need to locate the spot you wanted to copy'n'paste initially -- insofar it is not as comfortable as direct copy'n'paste. But it works more reliably....

It also works with acroread on Linux (but you have to choose 'Save as text...' from the file menu).

Update

You can use the pdffonts command line utility to get a quick-shot analysis of the fonts used by a PDF.

Here is an example output, which demonstrates where a problem for text extraction will very likely occur. It uses one of these hand-coded PDF files from a GitHub-Repository which was created to provide PDF sample files which are well commented and may easily be opened in a text editor:

$ pdffonts  textextract-bad2.pdf
  name                            type         encoding    emb sub uni object ID
  ------------------------------- ------------ ----------- --- --- --- ---------
  BAAAAA+Helvetica                TrueType     WinAnsi     yes yes yes     12  0
  CAAAAA+Helvetica-Bold           TrueType     WinAnsi     yes yes no      13  0

How to interpret this table?

• The above PDF file uses two subsetted fonts (as indicated by the BAAAAA+ and CAAAAA+ prefixes to their names, as well as by the yes entries in the sub column), Helvetica and Helvtica-Bold.
• Both fonts are of type TrueType.
• Both fonts use a WinAnsi encoding (a font encoding maps char identifiers used in the PDF source code to glyphs that should be drawn). However, only for font /Helvetica there is a /ToUnicode table available inside the PDF (for /Helvetica-Bold there is none), as indicated by the yes/no in the uni-column).

The /ToUnicode table is required to provide a reverse mapping from character identifiers/codes to characters.

A missing /ToUnicode table for a specific font is almost always a sure indicator that text strings using this font cannot be extracted or copied'n'pasted from the PDF. (Even if a /ToUnicode table is there, text extraction may still pose a problem, because this table may be damaged, incorrect or incomplete -- as seen in many real-world PDF files, and as also demonstrated by a few companion files in the above linked GitHub repository.)

Answer 2

If are able to successfully select and copy the text in Adobe Reader -- indicated that the PDF does contain text objects -- but you can't paste the copied text into Notepad without it looking like a bunch of garbage characters, then the problem is probably related to the CMap that the selected text uses.

The PDF specification provides many options for the display of textual content and the related extraction of the text content. A CMap specifies the mapping from character codes to character selectors. The PDF spec outlines some predefined CMaps, but other CMaps can also be embedded.

My guess is that either the CMap for this text is corrupt or that the PDFBox library doesn't support this particular CMap. I suggest trying a different SDK just to see if you get any different results.