alt Hacker NewsA PDF is a command stream designed for rendering. Interpreting the command stream to get the positions of each glyph is deterministic and existing libraries (I use both pdf_oxide and lopdf) do that fine. Once you have glyph positions, you need to use various heuristics to reconstruct words, paragraphs, columns, headers and footers, etc. For example, in a patent document, there's two columns with a gutter in the middle of line numbers. If you interpret the document as having a single line, you'll get numbers mixed up with the text, which can throw off efforts to find particular phrases. PDF builders also insert all sorts of weird crap into the OCR layers that has to get normalized out.
It's just a pretty pedestrian data-munging problem where there's no closed form perfect solution and you have to use various heuristics to get the right result.
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Yeah that’s why heuristics should work on the lowest possible layer, not on pdftotext. If you use pdftotext you’re stripping positional data and other stuff.
Do you use a public set of documents? I bet I could almost oneshot this with my harness :p
>It's just a pretty pedestrian data-munging problem
In other words, how complicated could it be?
Well, if Adobe has been introducing complications and making the format brittle and inflexible over the last 35 years to make it hard for its competitors to write software to process PDF files, quite complicated.
Yes, but the edge cases are infinite and so heuristics don't scale well. As an example, at some point you would likely find yourself with "dueling" heuristics, forcing you to tune them, which is brittle, or find yet another heuristic as a tie-breaker, which ratchets up the complexity. (I just spent a lot of time on an adjacent but much simpler problem before finally giving up on churning heuristics!)
As an example, many times it is impossible to determine the order of some words from just position data without considering the meanings of those words. This is why LLMs / VLMs are so much better at this task, because they can look at the document holistically like we can.
Also, funny that you mention patents, something I've worked on in the past as well! If you're looking only at US Patents, the USPTO data resource is much, much better: https://data.uspto.gov/home -- they provide the text in XML format (https://www.uspto.gov/learning-and-resources/xml-resources) which is also pretty complex but wayyyy easier to parse than PDFs!