not readable PDF files

Question

Hello,

I am writing 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 semanticaly correct chars or strings of digits and letters)

Could anybody help me??? THX :)

Answer 1

PDF is not a text document. It's more of a vector graphic format that sometimes can contain text. So there are some documents from which you can't extract text unless you are willing to do OCR. That's just the way it is.

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.

Answer 3

What was the PDF created with. Some PDFs do not contain any encoding information, just the data to draw it. So there is no way to extract the data.