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So you want to modify the text of a PDF by hand (2020)

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Re: So you want to modify the text of a PDF by hand (2020)

#11

Post by blincoln »

The PDF specification is wild. My current favourite trivia is that it supports all of Photoshop's layer blend modes for rendering overlapping elements.[1] My second-favourite is that it supports appended content that modifies earlier content, so one should always look for forensic evidence in all distinct versions represented in a given file.[2]

It's also a fun example of the futility of DRM. The spec includes password-based encryption, and allows for different "owner" and "user" passwords. There's a bitfield with options for things like "prevent printing", "prevent copying text", and so forth,[3] but because reading the document necessarily involves decrypting it, one can use the "user" password to open an encrypted PDF in a non-compliant tool,[4] then save the unencrypted version to get an editable equivalent.

[1] "More than just transparency" section of https://blog.adobe.com/en/publish/2022/01/31/20-years-of-tra...

[2] https://blog.didierstevens.com/2008/05/07/solving-a-little-p...

[3] Page 61 of https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandard...

[4] For example, a script that uses the pypdf library.

Re: So you want to modify the text of a PDF by hand (2020)

#12

Post by rhaway84773 »

aleden wrote: post #9

I'm surprised no one has mentioned qpdf. https://qpdf.readthedocs.io/en/stable/overview.html It turns a PDF (typically everything in it is compressed binary blobs) into a mixed binary/ASCII file (which itself is a PDF) that can be edited with vim.

It’s mentioned in the gist

> To view the compressed data, you can use a command line tool called qpdf.

Re: So you want to modify the text of a PDF by hand (2020)

#13

Post by chrnola »

aleden wrote: post #9

I'm surprised no one has mentioned qpdf. https://qpdf.readthedocs.io/en/stable/overview.html It turns a PDF (typically everything in it is compressed binary blobs) into a mixed binary/ASCII file (which itself is a PDF) that can be edited with vim.

The linked article literally mentions qpdf within the first few paragraphs.

Re: So you want to modify the text of a PDF by hand (2020)

#14

Post by userbinator »

ks2048 wrote: post #8

This seems to be missing an important point: at the end of PDF is a table ("cross-reference" table) that stores the BYTE-OFFSET to different objects in the file. If you modify things within the file, typically these offsets will change and the file will be corrupt. It looks like in this article, maybe they were only interested in changing one number to another, so none of the positions change. But, generally, adding/…

That's the weirdest part of the PDF spec IMHO. It's a mix of both binary and text, with text-specified byte offsets. It would be very interesting to read about why the format became like that, if its authors would ever talk about it. My guess is that it was meant to be completely textual at first (but then requiring the xref table to have fixed-length entries is odd), and then they decided binary would be more efficient.

Re: So you want to modify the text of a PDF by hand (2020)

#15

Post by ShadowBanThis01 »

anon____ wrote: post #7
gcanyon wrote: post #3

Earlier quoted context omitted.

I’m interested in extracting the contents of a pdf form — many individual text boxes. You’re saying libre office would likely be able to parse that pdf into a usable format?

With LibreOffice Draw you can edit the PDF (modify the text, move or change images, etc), then save as pdf, but it can't parse and save it as .odt, .doc, .html or similar.

LibreOffice has some really perplexing functionality gaps.

The one that baffles me is that it doesn't understand its own graphics format, so you have to export drawings to TIFF or something (if I remember correctly).

Re: So you want to modify the text of a PDF by hand (2020)

#16

Post by userbinator »

blincoln wrote: post #11

The PDF specification is wild. My current favourite trivia is that it supports all of Photoshop's layer blend modes for rendering overlapping elements.[1] My second-favourite is that it supports appended content that modifies earlier content, so one should always look for forensic evidence in all distinct versions represented in a given file.[2] It's also a fun example of the futility of DRM. The spec includes passwo…

In the context of a format that was originally proprietary and not widely available to everyone, and conceived in an era where encryption was strongly controlled by export law, that sort of security-by-obscurity was very common. Incidentally, a popular cracking tutorial back then was to de-DRM the official reader by patching the function that checks those permissions.

Re: So you want to modify the text of a PDF by hand (2020)

#17

Post by enriquto »

You can do this:

    pdf2ps a.pdf    # convert to postscript "a.ps"
    vim a.ps        # edit postscript by hand
    ps2pdf a.ps     # convert back to pdf
Some complex pdf (with embedded javascript, animations, etc) fail to work correctly after this back and forth. Yet for "plain" documents this works alright. You can easily remove watermarks, change some words and numbers, etc. Spacing is harder to modify. Of course you need to know some postscript.

Re: So you want to modify the text of a PDF by hand (2020)

#18

Post by mistrial9 »

esafak wrote: post #10
aidos wrote: post #5

This topic comes up periodically as most people think PDFs are some impenetrable binary format, but they’re really not. They are a graph of objects of different types. The types themselves are well described in the official spec (I’m a sadist, I read it for fun). My advice is always to convert the pdf to a version without compressed data like the author here has. My tool of choice is mutool (mutool clean -d in.pdf ou…

It is a shame Adobe designed a format so hard to work with that people are amazed when someone accomplishes what should be a basic task with it. Their design philosophy of creating a read-only format was flawed to begin with. What's the first feature people are going to ask for??

.. waves to Leonard Rosenthol

Re: So you want to modify the text of a PDF by hand (2020)

#19

Post by detourdog »

userbinator wrote: post #14
ks2048 wrote: post #8

This seems to be missing an important point: at the end of PDF is a table ("cross-reference" table) that stores the BYTE-OFFSET to different objects in the file. If you modify things within the file, typically these offsets will change and the file will be corrupt. It looks like in this article, maybe they were only interested in changing one number to another, so none of the positions change. But, generally, adding/…

That's the weirdest part of the PDF spec IMHO. It's a mix of both binary and text, with text-specified byte offsets. It would be very interesting to read about why the format became like that, if its authors would ever talk about it. My guess is that it was meant to be completely textual at first (but then requiring the xref table to have fixed-length entries is odd), and then they decided binary would be more effici…

I actually was at a Acrobat/PDF launch event in midtown NYC. It was an embedded file type that could be generated at the type of publishing and all dependencies could either be embedded or not.

This made a coherent point in a digital workflow that could be saved and reprinted with ease. This was a big deal before the portable document format came to be.

I once made a workflow that took pdf files from Word, filemaker, excel, and mini-cad. This all got combined into a single 9,000 page pdf. The final pdf had a coherent thumbnails, page numbers and headers and footer.

Only took a couple of hours to get the final documnet after pushing the go buttton.

Re: So you want to modify the text of a PDF by hand (2020)

#20

Post by aidos »

ks2048 wrote: post #8

This seems to be missing an important point: at the end of PDF is a table ("cross-reference" table) that stores the BYTE-OFFSET to different objects in the file. If you modify things within the file, typically these offsets will change and the file will be corrupt. It looks like in this article, maybe they were only interested in changing one number to another, so none of the positions change. But, generally, adding/…

In my experience it’s easiest just to break the xref table and run something like “mutool clean” to fix it again. It can be completely derived from the content so it’s safe to do.
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