As a library

The engine is the nupdf crate; the CLI is a thin layer over it. Add it to a project’s Cargo.toml, from crates.io once 0.1.0 is published:

[dependencies]
nupdf = "0.1"

or from the repository:

[dependencies]
nupdf = { git = "https://github.com/raspao-studio/nupdf" }

This program opens a file, extracts the structured text of every page on four threads, searches each page, and prints what the file got wrong:

use nupdf::{Document, parallel, search, stext};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let path = std::env::args().nth(1).ok_or("usage: scan <file.pdf>")?;
    let doc = Document::open(std::fs::read(path)?)?;
    let count = doc.page_count().map_err(|e| e.0)?;
    let pages: Vec<i32> = (0..count).collect();

    // Structured text of every page, on up to 4 threads, in page order.
    let opts = stext::Options::default();
    let results = parallel::extract(&doc, &pages, opts, Some(4));
    for (i, page) in results.into_iter().enumerate() {
        let page = page?;
        let hits = search::search_page(&page, "hello", search::options::IGNORE_CASE)?;
        println!("page {}: {} hit(s)", i + 1, hits.len());
        print!("{}", page.text());
    }

    // What was wrong with the file, as data rather than as a failure.
    for d in doc.diagnostics().snapshot() {
        eprintln!("{}: {}", d.code, d.message);
    }
    Ok(())
}

A few things to know:

• A Document is shared across threads. It is parsed once and pages are worked on at once; parallel::extract with None instead of Some(4) runs the pages one after another, as MuPDF does, with the same results.
• Pages are 0-based in the library (doc.page(0) is the first), 1-based in the CLI.
• Limits. Document::open uses the default Limits; Document::open_with(data, limits) sets the largest decoded stream, xref table and the rest, as the CLI’s --max-* options do.
• Diagnostics are data. A damaged file opens if MuPDF would open it, and what nupdf repaired or skipped is in doc.diagnostics(), with the same stable codes the CLI reports.
• The modules follow MuPDF’s. stext is fz_stext, search is MuPDF’s search, redact, write, form and the rest likewise; the architecture maps each module to the MuPDF file it comes from.

The library’s API is not yet stable: until 1.0 a minor release may change it. The CLI’s JSON reports are the stable interface, versioned per schema (agent interface).