Understanding the PDF File Format

15 Replies to “Understanding the PDF File Format”

1. Hello, JPedal team 🙂
   First of all, allow me to express my satisfaction for reading such simple and clear PDF knowledges background. This is really helpful for who just starts into the PDF world. Thank you for this great job.
   I found these posts while looking for a *real* way of doing some redacts. I thought I had everything when I found ‘pdfedit’ (with a combination of its ‘replaceText’, its ‘findText’, its ‘drawRect’ and its ‘flattener’ functionalities), but then the ugly truth came up: sometimes not all characters are available. I guess it’s embedded fonts fault, but I am not quite sure. Here is when I started to read your posts 😉
   The fact is, I guess I will be able to implement a functional redact feature if found the way to ensure my replacement char (the one I use to replace each char of the redacted phrase) is in fact available. I see two scenarios here:
   1- the char is available (and so, everybody is happy :P), or
   2- the char is not available, but I am able to insert it to the right (embedded?) font.
   Could you help me to accomplish this, please? Some hints could be appreciated.
   Thanks in advance.
   Best regards and keep this spirit!

   —
   Alejandro

2. Hello Leon,
   thanks for your work here. This is a really nice collection of helpful hints and tips. I’m searching the web looking for some explaining word on how to embed a XML file to a PDF/a3 file by code. Can you help me?
   Kind regards,
   Al Mudy

   1. We recommend Itext for embedding data in PDF.

3. I have a question.
   What is the data format used in PDF to draw table. Is there some type of native table object in PDF we can use or is it just a vector graphic that paints a table. and how table extraction done in PDF content extraction libraries.
   Could you please explain this.

   1. If the PDF was created with additional tagged meta data then there may be tags (there is no specification for these so they might be HTML or some custom user creation). Most files do not have this feature enabled so I am afraid it is usually just content painted in an arbitrary order which your brain then interprets as a table.

4. Hello, JPedal team ????

   Thanks for sharing your knowledge. The articles help me a lots.

5. Hi Thanks for the excellent guide it helped me to understand this much better.

   I have a question though. Some PDF’s which I read using the PDFSharp Library for Visual Studio. When I grab the text from a page.

   I get weird text.

   Instead of it being in clear text it seems to be encoded or possibly encrypted
   I.E
   Td
   (/0\(11$2#\(11#$2’3#45’67″$8) Tj

   Reading Chapter 9 of PDF 32000-1:2008 I cant gather if this is a font encoding or not.
   How can I go about decoding the above text ?

6. You need to stop thinking of it as text. It is encoded binary content which may look like test if the PDF uses WIN encoding.

   There ate lots of articles to help you on our blog like

   https://blog.idrsolutions.com/2011/03/understanding-the-pdf-file-format-–-pdf-text-extraction-with-java/
   https://blog.idrsolutions.com/2011/04/understanding-the-pdf-file-format-–-custom-font-encodings/

7. A pdf file was edited using iTextPdf programmatically. The PDF has a few radio buttons. I can see the resulting pdf file in Chrome browser without issues, However when I open the same file in Acrobat Reader, radio buttons are not showing up in the file.
   What is missing here?

8. It could be anything. I would recommend using the excellent free Itext tool Rups to look at the values set.

9. Hello Mark, your blogs are amazing. I am looking for some technical to develop a software that can edit PDF files (text and images), like Foxit Reader. Have you and your colleagues done this, can you give me some advice. Thank you and Best Regards

   1. Many thanks for the compliment! I will pass on your feedback to the team. To answer your question we had considered in the past but have chosen to focus on our current product range for developers.

10. Hello Mark – at my last job as a software engineer, I worked with a third party PDF parser (PDFLib) to extract metadata (fonts, colors, page size, document info, etc.) – and arrived at a pretty good Java API to retrieve detailed metadata in json and protobuf format. Unfortunately the tool I was using could not extract page background color, at least according to the vendor when we contacted them about this. I was incredulous that such a simple attribute as page background color could not be retrieved. Anyway, now that I have retired (that was my last full time job), I plan to see if I can crack this issue in my own time – obviously I don’t have access to the source code I developed at my last job but all I really want to do for now is to discover how the PDF specification represents page background color, chose a parser that allows me to extract that, and write a simple demo utility that extracts that.
    Any thoughts? Thanks in advance.

    1. Hi Chris,

       It is certainly an interesting problem. What do you mean by page background colour? There is no global setting for this value and the final colour of any pixel will depend on parsing and executing the PDF commands – the easiest way to get this is to rasterize the page so you get the end result, unless you mean colour behind text. So it is not a simple task.

       As regards recommending Open Source PDF libraries, it really depends what language you want to use. You have PDF.js (Javascipt), PdfBox/iText (Java) or xpdf (C).

11. Thanks for that Mark. I wasn’t aware that there is no single setting, although of course it’s easy to set from Acrobat. Regarding rasterizing the PDF, I will certainly look into that, but yes the problem the software was trying to address was to detect ‘hidden’ text: that is text with identical color as its background.

    Likely I will be using Java or even Python (juts for fun!)

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