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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)

#31

Post by pavlov »

aardvark179 wrote: post #25
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…

Aren’t the blend modes supported just the Porter-Duff compositing modes? You might think that’s overkill, but it’s a really good mapping of what other rendering pipelines offer and it can really help reduce the work to produce a PDF.

The original Porter-Duff compositing operators don’t cover Photoshop-style blending. Here’s a link with pictures:

http://ssp.impulsetrain.com/porterduff.html

The Porter-Duff operators are appealingly rigorous and easy to implement because they’re simply the possible combinations of a simple formula. But many of these operators are not very useful either.

The Photoshop blending modes are practically the opposite: they are not derived from anything mathematically appealing, it’s really just a collection of algorithms that Photoshop’s designers originally found useful. They reflect the limitations of their early 1990s desktop computer implementations (for example, no attempt is made to account for gamma correction when combining the layers, which makes many of these operations behave very differently from actual light that they mean to emulate).

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

#33

Post by jl6 »

gpvos wrote: post #27
jl6 wrote: post #21

This seems to be missing an important step in the use of qpdf’s --qdf mode: after you’ve finished editing, you need to run the file through the fix-pdf utility to recalculate all the object offsets and rebuild the cross-reference table that lives at the end of the file (unless you only change bytes in-place rather than adding or removing bytes). My top 3 fun PDF facts: 1) Although PDF documents are typically 8-bit bi…

After you've finished editing, just run it through qpdf without parameters, as explained in the beginning of the article, and it will recompress the data and recreate the xref table. No need for yet another tool.

I guess you could, but this is the source of the errors (actually warnings) that the article mentions. Probably best to fix the file with the provided tool (fix-qdf is distributed with qpdf) rather than get in the habit of ignoring warnings.

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

#34

Post by miki123211 »

What people often miss about PDF is that it's closer to an image format in some ways than to a Word document. Word documents, PDFs and images are in document editing what DAW projects, midis and mp3 files are in music and what Java source code, JVM bytecode and pure x86 machine code are in software.

The primary purpose of a PDF file is to tell you what to display (or print), with perfect clarity, in much fewer bytes than an actual image would take. It exploits the fact that the document creator knows about patterns in the document structure that, if expressed properly, make the document much more compressible than anything that an actual image compression algorithm could accomplish. For example, if you have access to the actual font, it's better to say "put these characters at these coordinates with that much spacing between them" than to include every occurrence of every character as a part of the image, hoping that the compression algorithm notices and compresses away the repetitions. Things like what character is part of what word, or even what unicode codepoint is mapped to which font glyph are basically unimportant if all you're after is efficiently transferring the image of a document.

If you have an editable document, you care a lot more about the semantics of the content, not just about its presentation. It matters to you whether a particular break in the text is supposed to be multiple spaces, the next column in a table or just a weird page layout caused by an image being present. If you have some text at the bottom of each page, you care whether that text was put there by the document author multiple times, or whether it was entered once and set as a footer. If you add a new paragraph and have to change page layout, it matters to you that the last paragraph on this page is a footnote and should not be moved to the next one. If a section heading moves to another page, you care about the fact that the table of contents should update automatically and isn't just some text that the author has manually entered. If you're a printer or screen, you care about none of these things, you just print or display whatever you're told to print or display. For a PDF, footnotes, section headings, footers or tables of contents don't have to be special, they can just be text with some meaningless formatting applied to it. This is why making PDF work for any purpose which isn't displaying or printing is never going to be 100% accurate. Of course, there are efforts to remedy this, and a PDF-creating program is free to include any metadata it sees fit, but it's by no means required to do so.

This isn't necessarily the mental model that the PDF authors had in mind, but it's an useful way to look at PDF and understand why it is the way it is.

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

#35

Post by eschaton »

Anybody trying to do this is missing the point of PDF: It’s a page-description format and therefore only represents the marks on a page, not document structure.

One should not attempt to edit a PDF, one should edit the document from which the PDF is generated.

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

#36

Post by pmarreck »

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…

The roots of PDF are PostScript, which is like Forth, and is text-based, so that’s why

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

#37

Post by pwg »

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??

> It is a shame Adobe designed a format so hard to work with

PDF was not designed to be editable, nor for anyone to "work with" it in that way.

It was designed (at least the original purpose circa 1989) to represent printed pages electronically in a format that would view and print identically everywhere. In fact, the initial advertising for the "value" of the PDF format was exactly this, no matter where a recipient viewed your PDF output, it would look, and print, identically to everywhere else.

It was originally meant to be "electronic paper".

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

#38

Post by lucb1e »

eschaton wrote: post #35

Anybody trying to do this is missing the point of PDF: It’s a page-description format and therefore only represents the marks on a page , not document structure . One should not attempt to edit a PDF, one should edit the document from which the PDF is generated.

I'll stop trying to edit PDFs when people stop sending me PDFs that I want to edit.

Somehow it became "unprofessional" to just send meant-to-be-editable documents around for everyone to enjoy, so this is where we end up...

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

#39

Post by lucb1e »

rogeliodh wrote: post #32

LibreOffice can open and edit PDFs. Last time I tried it was really good. Not sure what limitations are there.

For me it always seems to change the font from whatever was built into the PDF (rendered just fine in any PDF reader) to a random system font which completely breaks the spacing, making different parts of the document overflow into each other

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

#40

Post by louthy »

eschaton wrote: post #35

Anybody trying to do this is missing the point of PDF: It’s a page-description format and therefore only represents the marks on a page , not document structure . One should not attempt to edit a PDF, one should edit the document from which the PDF is generated.

> It’s a page-description format and therefore only represents the marks on a page, not document structure

Maybe they should have called it ‘Page Description Format’ then? Instead of ‘Portable Document Format’

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