Idiomatic Rust bindings for Pdfium

pdfium-render provides an idiomatic high-level Rust interface around the low-level bindings to Pdfium exposed by the excellent pdfium-sys crate.

``` // Iterates over each page in the given test PDF file and renders each page // as a separate JPEG image in the current working directory.

use pdfium_render::{Pdfium, PdfBitmapConfig, PdfBitmapRotation, PdfiumError};
use image::ImageFormat;

fn export_pages() -> Result<(), PdfiumError> {
    let document = Pdfium::new(Pdfium::bind_to_system_library()?)
        .load_pdf_from_file("test.pdf", None)?;

    for page in document.pages() {
        page.get_bitmap_with_config(&PdfBitmapConfig::new()
                .set_target_width(2000)
                .set_maximum_height(2000)
                .rotate_if_landscape(PdfBitmapRotation::Degrees90, true))?
            .as_image() // Renders this page to an Image::DynamicImage
            .as_bgra8()?
            .save_with_format(format!("test-page-{}.jpg", page.index()), ImageFormat::Jpeg)?;
    }

    Ok(())
}

```

In addition to providing a more natural interface to Pdfium, pdfium-render differs from pdfium-sys in several other important ways:

pdfium-render uses libloading to late bind to a Pdfium library at run-time, whereas pdfium-sys binds to a library at compile-time. Not only can compile-time binding be a bit fiddly to configure, but it precludes compiling pdfium-sys to WASM. By binding to Pdfium at run-time instead of compile-time, pdfium-render can dynamically switch between bundled libraries and system libraries and offer idiomatic Rust error handling in situations where a Pdfium library is not available.
Late binding to Pdfium means that pdfium-render can be compiled to WASM for running in a browser; this is not possible with pdfium-sys.
Pages rendered by Pdfium can be exported as instances of Image::DynamicImage for easy, idiomatic post-processing.

Porting existing Pdfium code from other languages

The high-level idiomatic Rust interface provided by the Pdfium struct is entirely optional; the Pdfium struct wraps around raw FFI bindings defined in the PdfiumLibraryBindings trait, and it is completely feasible to simply use those raw FFI bindings directly rather than the high level interface. This makes porting existing code that calls FPDF_* functions very straight-forward, while still gaining the benefits of late binding and WASM compatibility. For instance, the following code snippet (taken from a C++ sample):

``` string test_doc = "test.pdf";

FPDF_InitLibrary();
FPDF_DOCUMENT doc = FPDF_LoadDocument(test_doc, NULL);
// ... do something with doc
FPDF_CloseDocument(doc);
FPDF_DestroyLibrary();

```

would translate to the following Rust code:

``` let bindings = Pdfium::bindtosystem_library()?;

let test_doc = "test.pdf";

bindings.FPDF_InitLibrary();
let doc = bindings.FPDF_LoadDocument(test_doc, None);
// ... do something with doc
bindings.FPDF_CloseDocument(doc);
bindings.FPDF_DestroyLibrary();

```

External Pdfium builds

pdfium-render does not bundle Pdfium at all. You can either bind to a system-provided library or package an external build of Pdfium alongside your Rust application. When compiling to WASM, packaging an external build of Pdfium as a WASM module is essential.

Native builds of Pdfium for all major platforms: <https://github.com/bblanchon/pdfium-binaries/releases>
WASM builds of Pdfium, suitable for deploying alongside pdfium-render: <https://github.com/paulo-coutinho/pdfium-lib/releases>

Compiling to WASM

See <https://github.com/ajrcarey/pdfium-render/tree/master/examples> for a full example that shows how to bundle a Rust application using pdfium-render alongside a pre-built Pdfium WASM module for in-browser introspection and rendering of PDF files.

Development status

The initial focus of this crate has been on rendering pages in a PDF file; consequently, FPDF* functions related to bitmaps and rendering have been prioritised. By 1.0, the functionality of all FPDF* functions exported by Pdfium will be available.

If you need a function that is not currently exposed, just raise an issue.