如何从这个压缩的 PDF/A 中提取文本？

Question

For machine learning purposes ( sckit-learn ), I need to extract the raw text from lots of PDF files. First off, I was using xpdf pdftotext to do this task:

exe = r'"'+os.path.join(xpdf_path,"pdftotext.exe")+'"'
cmd = exe+" "+"\""+pdf+"\""+" "+"\""+pdf+".txt"+"\""
subprocess.check_output(cmd)
with open(pdf+".txt") as f:
    texto_converted = f.read()

But unfortunately, for few of them, I was unable to get the text because they are using "stream" on their pdf source, like this one .

The result is something like this:

59!"#$%&'()*+,-.#/#01"21"" 345667.0*(879:4$;<;4=<6>4?$@"12!/ 21#$@A$3A$>@>BCDCEFGCHIJKIJLMNIJILOCNPQRDS QPFTRPUCTCVQWBCTTQXFPYTO"21 "#/!"#(Z[12\&A+],$3^_3;9`Z &a# .2"#.b#"(#c#A(87*95d$d4?$d3e#Z"f#\"#2b?2"#`Z 2"!eb2"#H1TBRgF JhiO
jFK# 2"k#`Z !#212##"elf/e21m#*c!n2!!#/bZ!#2#`Z "eo ]$5<$@;A533> "/\ko/f\#e#e#p

I Even trying using zlib + regex:

import re
import zlib

pdf = open("pdfa.pdf", "rb").read()
stream = re.compile(b'.*?FlateDecode.*?stream(.*?)endstream', re.S)

for s in re.findall(stream,pdf):
    s = s.strip(b'\r\n')
    try:
        print(zlib.decompress(s).decode('UTF-8'))
        print("")
    except:
        pass

The result was something like this:

1 0 -10 -10 10 10 d1
0.01 0 0 0.01 0 0 cm
1 0 -10 -10 10 10 d1
0.01 0 0 0.01 0 0 cm

I even tried pdftopng (xpdf) to try tesseract after, without success So, Is there any way to extract pure text from a PDF like that using Python or a third party app?

Answer 1

If you want to decompress the streams in a PDF file, I can recommend using qdpf , but on this file

 qpdf --decrypt --stream-data=uncompress document.pdf out.pdf

doesn't help either.

I am not sure though why your efforts with xpdf and tesseract did not work out, using image-magick's convert to create PNG files in a temporary directory and tesseract , you can do:

import os
from pathlib import Path
from tempfile import TemporaryDirectory
import subprocess

DPI=600

def call(*args):
    cmd = [str(x) for x in args]
    return subprocess.check_output(cmd, stderr=subprocess.STDOUT).decode('utf-8')

def ocr(docpath, lang):
    result = []
    abs_path = Path(docpath).expanduser().resolve()
    old_dir = os.getcwd()
    out = Path('out.txt')
    with TemporaryDirectory() as tmpdir:
         os.chdir(tmpdir)
         call('convert', '-density', DPI, abs_path, 'out.png')
         index = -1
         while True:
             # names have no leading zeros on the digits, would be difficult to sort glob() output
             # so just count them
             index += 1
             png = Path(f'out-{index}.png')
             if not png.exists():
                 break
             call('tesseract', '--dpi', DPI, png, out.stem, '-l', lang)
             result.append(out.read_text())
         os.chdir(old_dir)
    return result

pages = ocr('~/Downloads/document.pdf', 'por')
print('\n'.join(pages[1].splitlines()[21:24]))

which gives:

DA NÃO REALIZAÇÃO DE AUDIÊNCIA DE AUTOCOMPOSIÇÃO NO CASO EM CONCRETO

Com vista a obter maior celeridade processual, assim como da impossibilidade de conciliação entre

If you are on Windows, make sure your PDF file is not open in a different process (like a PDF viewer), as Windows doesn't seem to like that.

The final print is limited as the full output is quite large.

This converting and OCR-ing takes a while so you might want to uncomment the print in call() to get some sense of progress.

Answer 2

There are two fairly simple techniques you can use.

1) Google's "Tessaract" open source OCR (optical character recognition). You could apply this evenly to all PDFs, though converting all that data into pixels and then working magic upon them is going to be more computationally expensive. Which is more important, engineer time or CPU time? There's a pytesseract module. Note that this tool works on image formats, so you'd have to use something like GhostScript (another open source project) to convert all of a PDF's pages to images, then run [py]tessaract on those images.

2) pyPDF can get each page and programmatically extract any text draw operations in the order they were drawn onto the page . This may be nothing like the logical reading order of the page... While a PDF could draw all the 'a's and then all the 'b's (and so forth), it's actually more efficient to draw everything in "font a", then everything in "font b". It's important to note that "font b" might just be the italic version of "font a". This produces a shorter/more efficient stream of drawing commands, though probably not by such an amount as to be a good business decision to do so.

The kicker here is that a random pile of PDF files might require you to do some OCR. A poorly assembled PDF (one with a font subset that has no "to unicode" data) can't be properly mined for text even though it has nothing but text drawing operations. "Draw glyphs one through five from "font C" doesn't mean much if you don't know that those first five glyphs are "glyph", because that's the order they were used in.

On the other hand, if you've got home-grown PDFs or all your pdfs are from some known source (Word's pdf converter for example), you'll know what to expect in advance.

Note that the only thing mentioned above that I've actually used is Ghostscript. I remember it having a solid command line interface we used to generate images for some online PDF viewer Many Years Ago.