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How to compare two PDF documents

95 points| ingve | 4 years ago |eclecticlight.co | reply

30 comments

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[+] btown|4 years ago|reply
https://draftable.com/compare is by far the best solution I've found for this, and it's a shame it's not more widely known about. It's not open-source, and their offline app is Windows only, but its ability to handle multi-page relayouts is far and above Acrobat's diff functionality (as the OP laments), and there's a free online version that's reasonably secure so long as you don't share the secret URL around. I've used it many times to obtain readable redlines when only given successive "baked" versions of a document, and it's a really useful tool for any B2B startup founder.
[+] phiresky|4 years ago|reply
There's a free and opensource program called `diffpdf` that can compare both visually and by text. It has a GUI and works great, though it doesn't specially handle layout changes. It's included in the normal package sources in most Linux distros:

    apt install diffpdf
    pacman -S diffpdf
https://gitlab.com/eang/diffpdf
[+] upofadown|4 years ago|reply
It's logically the same issue as with signing documents. You have to decide what aspect of the document you want to certify and ignore the rest. If you sign something in a complex document format you don't even know exactly what you are signing. Much of what you are signing is not even visible.

Things like legal documents should be restricted to plain text... and stuff like line endings should be standardized for this purpose.

[+] wolverine876|4 years ago|reply
> line endings should be standardized

I don't see that happening! ;)

[+] mckmk|4 years ago|reply
Just to throw in another solution for anyone looking. Abbyy FineReader has a comparison module that is excellent. https://pdf.abbyy.com/how-to/compare-documents/
[+] fmos|4 years ago|reply
Second that. This is THE solution for comparing any two PDFs (image or not). I’ve been using it for years almost on a daily basis. Part of its use is certainly derived from the excellent OCR engine it relies on. Also, this runs fully local, which is critical for legal purposes. (edit for context: I still use v14)
[+] redman25|4 years ago|reply
For visually comparing PDFs instead of textually comparing them, I use https://parepdf.com I work in publishing, so comparing printer proof PDFs is something we do regularly.
[+] seesawtron|4 years ago|reply
It just seems to overlay two pdfs' text and images in two colors. Looks kind of ugly but atleast its a free service.
[+] Toutouxc|4 years ago|reply
A guy I work with did his PDF diffing (basically testing whether invoices generated by new code match those generated by old code) by running the two invoices through ImageMagick and subtracting them from each other, basically looking for different pixels (maybe > $threshold) and then looking at the visual diff with missing pixels brightly colored. I thought that was really elegant.
[+] yardshop|4 years ago|reply
BeyondCompare from Scooter Software [1] does a good job of comparing PDFs, although it does only compare the extracted text. But that is just one of the many many things it can do, so $60 (for the pro version or $30 for standard) gets you a lot more than just PDF comparison.

[1] https://www.scootersoftware.com

[+] riobard|4 years ago|reply
Ignoring all the bad things about Microsoft Word, it really nails revision management for non-tech people. I've worked thru many legal contracts, collaborating with lawyers and other parties. I wish there'd be better alternatives, tho.
[+] umvi|4 years ago|reply
Store PDFs as html or markdown under the hood which is diffable and only generate PDF at the last second
[+] wolverine876|4 years ago|reply
Nice if you are the only user. If you distribute documents to others, or receive them from others, it's less practical.

Also, Markdown won't nearly represent all the formatting possibilities of a PDF.

Also, I'm not sure HTML is much easier to compare than PDF. Both are markup languages, essentially.

But following along your idea, it would be nice if the document itself contained separate content and presentation layers, so that we could manipulate and analyze (and compare) the content.

[+] spdegabrielle|4 years ago|reply
PDF is an abomination & a relic.
[+] wolverine876|4 years ago|reply
OK, so what do you recommend for readability and also for stable presentation across platforms?

Reading a book on a screen, I find PDF far superior to HTML or Word or text.

[+] sbmthakur|4 years ago|reply
What alternative do you suggest?
[+] Communitivity|4 years ago|reply
It's been a while, but if I remember right PDF is nothing more than PostScript with extensions by Adobe and a PDF pre-amble. Essentially, it is structured text based on PostScript with the possibility of added embedded binary streams for things such as forms. Given this, implementing a good compare tool should not be too difficult.

The hardest parts would probably be scoping to figure out what Adobe extensions to support, and acquiring or reverse engineering the Adobe extension formats.

Simple PDF example:

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  ( This is a small demonstration .pdf file - ) Tj
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  ( just for use in the Virtual Mechanics tutorials. More 
  text. And more ) Tj
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  ( text. And more text. And more text. And more text. ) Tj
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  more text. And ) Tj
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  ( more text. And more text. And more text. And more text. 
  And more text. ) Tj
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  ( And more text. And more text. ) Tj
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  ( And more text. And more text. And more text. And more 
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  ( text. Oh, how boring typing this stuff. But not as boring as watching ) Tj
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  ( paint dry. And more text. And more text. And more text. 
  And more text. ) Tj
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  69.2500 640.8000 Td
  ( Boring.  More, a little more text. The end, and just as 
  well. ) Tj
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[+] untoxicness|4 years ago|reply 

  #! /bin/bash

  pdf_one="$1"
  pdf_two="$2"

  text_one=$(mktemp)
  text_two=$(mktemp)

  pdftotext "$pdf_one" "$text_one"
  pdftotext "$pdf_two" "$text_two"

  diff "$text_one" "$text_two"
[+] halostatue|4 years ago|reply
It is entirely possible for that to present unexpected differences because of the way that the PDF format works. One can have two different PDFs that encode the same content in two different ways and unless `pdftotext` does virtual layout and then OCR-like extraction, you might end up with jumbled text or text in different orders.
[+] Hello71|4 years ago|reply
not only is this the first solution presented in the article, you've not even bothered to pass -layout to pdftotext or -u/-y to diff which would make this marginally workable. spaces after shebang also doesn't always work (e.g. in qemu), you don't clean up the temporary files, and the temporary files have unhelpful names.
[+] seoaeu|4 years ago|reply
Won't that fail terribly if the text has been re-flowed?