pdf2xml-viewer - A simple viewer and inspection tool for text boxes in PDF documents
July 2016 / Feb. 2017, Markus Konrad [email protected] / Berlin Social Science Center
Introduction
This is a small tool with which it is possible to view and examine individual text boxes in PDF documents. This is very helpful for analyzing the distribution of texts across a page, especially in the case of OCR-processed PDFs (so called "sandwich PDFs") from which you might want to extract structured information (see pdftabextract for this). With this viewer, you can examine such PDFs and have a look at the properties of individual text boxes, like position, width, height or font specification. In combination with pdftabextract, you can view the grids that were generated for the detected columns and rows. This blog post shows an example usage.
The viewer requires you to convert your PDFs to the pdf2xml format. Afterwards you can start up a local webserver, display this XML file in the viewer (as seen below) and examine the individual text boxes with your browsers developer console.
The created file in pdf2xml format can later also be used to extract structured information, which I explain in my series of blog posts about data mining PDFs.
How to use it
1. Convert a PDF to pdf2xml format
At first, you need to convert your PDFs using the poppler-utils, a package which is part of most Linux distributions
and is also available for OSX via Homebrew or MacPorts. From this package we need the command pdftohtml and can create
an XML file in pdf2xml format in the following way using the Terminal:
pdftohtml -c -hidden -xml input.pdf output.xml
The arguments input.pdf and output.xml are your input PDF file and the created XML file in pdf2xml format respectively. It is important that you specifiy the -hidden parameter when you're dealing with OCR-processed ("sandwich") PDFs. You can furthermore add the parameters -f n and -l n to set only a range of pages to be converted.
2. Start a minimal local webserver to display the text boxes in the PDF with the viewer
Now that you have your file(s) in pdf2xml format, change to the directory where pdf2xml-viewer resides (where it's index.html file is). You should also copy the generated XML files to this location. Now let's start up a minimal local webserver. This can be done very easily with Python, which is installed on Linux and Mac OSX by default. You can do so in the Terminal with Python 2.x:
python -m SimpleHTTPServer 8080
Or with Python 3:
python3 -m http.server 8080 --bind 127.0.0.1
Now you open your browser and go to the adress http://127.0.0.1:8080. The viewer shows up and you can now enter the file name of your file to load (it must be relative to the directory in which pdf2xml-viewer resides). If you just want to see an example, type in example/ocr-output.pdf.xml and load this file. Now you browse through the pages of your PDF document and you'll see the text boxes with red frames. You can further examine these boxes by using your browser's inspection tools (right click on element and select "Inspect" in Chrome or Firefox) as seen below:
3. Use the advanced features of the viewer
You can load a page grid JSON file that was generated with pdftabextract (function common.save_page_grids):
4. Extract data from your PDFs
If you want to extract structured data from the PDFs, you should have a look at the pdftabextract package.
Technical details
This viewer uses d3.js to display the pdf2xml file. I chose this approach because it is the fastest and simples in order to inspect individual elements of a (OCR-processed) PDF document, without using expensive special software. Furthermore it allows to add additional features such as displaying overlays of calculated lines or grids.
License
Apache License 2.0. See LICENSE file.